Update all curl documentation examples and references with new x-medusa-access-token header (#6326)

## What

This is to update incorrect documentation in regards to authentication to the Admin API - raised in https://github.com/medusajs/medusa/issues/6264.

## Why

Because the current documentation has been incorrect since the September 2023 release of [v1.17.0](https://github.com/medusajs/medusa/releases/tag/v1.17.0), which had breaking changes to API token usage.

## How

Simple search and replace. I was asked to replace occurrences under `www/apps/docs/content/` but there were also additional places where I thought references should also be updated:

- `packages/medusa/src/api/`
- `www/apps/api-reference/`

Feel free to revert them as needed.

There is also some inconsistency between the format shown in examples e.g. `<API_TOKEN>` vs `{api_token}` vs `{access_token}`.

I have kept the format the same in all cases as the original, as surrounding documentation text would not have format updated as well. I suggest maybe reviewing the documentation and keeping to a consistent format e.g. `<API_TOKEN>`.

 
## Testing 

I have not tested these changes. I would assume the `packages/medusa/src/api/` changes may need more thorough testing?

www/apps/docs/content/modules/customers/admin/manage-customer-groups.mdx

@@ -109,7 +109,7 @@ You can create a customer group by sending a request to the Create Customer Grou
 
     ```bash
     curl -L -X POST '<BACKEND_URL>/admin/customer-groups' \
-    -H 'Authorization: Bearer <API_TOKEN>' \
+    -H 'x-medusa-access-token: <API_TOKEN>' \
     -H 'Content-Type: application/json' \
     --data-raw '{
         "name": "VIP"
@@ -191,7 +191,7 @@ You can get a list of all customer groups by sending a request to the List Custo
 
     ```bash
     curl -L -X GET '<BACKEND_URL>/admin/customer-groups' \
-    -H 'Authorization: Bearer <API_TOKEN>'
+    -H 'x-medusa-access-token: <API_TOKEN>'
     ```
 
   </TabItem>
@@ -260,7 +260,7 @@ You can retrieve a single customer group by sending a request to the Get a Custo
 
     ```bash
     curl -L -X GET '<BACKEND_URL>/admin/customer-groups/<CUSTOMER_GROUP_ID>' \
-    -H 'Authorization: Bearer <API_TOKEN>'
+    -H 'x-medusa-access-token: <API_TOKEN>'
     ```
 
   </TabItem>
@@ -346,7 +346,7 @@ You can update a customer group’s data by sending a request to the Update Cust
 
     ```bash
     curl -L -X POST '<BACKEND_URL>/admin/customer-groups/<CUSTOMER_GROUP_ID>' \
-    -H 'Authorization: Bearer <API_TOKEN>' \
+    -H 'x-medusa-access-token: <API_TOKEN>' \
     -H 'Content-Type: application/json' \
     --data-raw '{
         "metadata": {
@@ -424,7 +424,7 @@ You can delete a customer group by sending a request to the Delete a Customer Gr
 
     ```bash
     curl -L -X DELETE '<BACKEND_URL>/admin/customer-groups/<CUSTOMER_GROUP_ID>' \
-    -H 'Authorization: Bearer <API_TOKEN>'
+    -H 'x-medusa-access-token: <API_TOKEN>'
     ```
 
   </TabItem>
@@ -520,7 +520,7 @@ You can add a customer to a group by sending a request to the Customer Group’s
 
     ```bash
     curl -L -X POST '<BACKEND_URL>/admin/customer-groups/<CUSTOMER_GROUP_ID>/customers/batch' \
-    -H 'Authorization: Bearer <API_TOKEN>' \
+    -H 'x-medusa-access-token: <API_TOKEN>' \
     -H 'Content-Type: application/json' \
     --data-raw '{
         "customer_ids": [
@@ -608,7 +608,7 @@ You can retrieve a list of all customers in a customer group using the List Cust
 
     ```bash
     curl -L -X GET '<BACKEND_URL>/admin/customer-groups/<CUSTOMER_GROUP_ID>/customers' \
-    -H 'Authorization: Bearer <API_TOKEN>'
+    -H 'x-medusa-access-token: <API_TOKEN>'
     ```
 
   </TabItem>
@@ -714,7 +714,7 @@ You can remove customers from a customer group by sending a request to the Remov
 
     ```bash
     curl -L -X DELETE '<BACKEND_URL>/admin/customer-groups/<CUSTOMER_GROUP_ID>/customers/batch' \
-    -H 'Authorization: Bearer <API_TOKEN>' \
+    -H 'x-medusa-access-token: <API_TOKEN>' \
     -H 'Content-Type: application/json' \
     --data-raw '{
         "customer_ids": [

www/apps/docs/content/modules/customers/admin/manage-customers.mdx

@@ -111,7 +111,7 @@ You can show a list of customers by sending a request to the [List Customers API
 
     ```bash
     curl -L -X GET '<BACKEND_URL>/admin/customers' \
-    -H 'Authorization: Bearer <API_TOKEN>'
+    -H 'x-medusa-access-token: <API_TOKEN>'
     ```
 
   </TabItem>
@@ -214,7 +214,7 @@ You can create a customer account by sending a request to the [Create a Customer
 
     ```bash
     curl -L -X POST '<BACKEND_URL>/admin/customers' \
-    -H 'Authorization: Bearer <API_TOKEN>' \
+    -H 'x-medusa-access-token: <API_TOKEN>' \
     -H 'Content-Type: application/json' \
     --data-raw '{
         "email": "<EMAIL>",
@@ -314,7 +314,7 @@ You can edit a customer’s information by sending a request to the [Update a Cu
 
     ```bash
     curl -L -X POST '<BACKEND_URL>/admin/customers/<CUSTOMER_ID>' \
-    -H 'Authorization: Bearer <API_TOKEN>' \
+    -H 'x-medusa-access-token: <API_TOKEN>' \
     -H 'Content-Type: application/json' \
     --data-raw '{
         "first_name": "<FIRST_NAME>"