chore: Update admin build/serve configuration (#9584)

**Breaking changes**

The `outDir` has been deprecated and wont be used anymore, instead all the path are computed internally following these rules
- if admin is not `disabled` and the `build` command is run without the `--admin-only` flag, then the admin output dir will be `.medusa/server/public/admin` and it will be served from that same location from the medusa instance.
- if admin is not `disabled` and the `build` command is run with the `--admin-only` flag, then the admin output dir will be `.medusa/admin` with the purpose of deploying the admin separately. ⚠️ (expect to receive a warning log)
- if the admin is `disabled` and the `build` command is run with the `--admin-only` flag, then fallback to rule number 2

| admin enabled  | medusa build --admin-only  | output dir |
|---|---|---|
| true  | true  | `.medusa/admin` ⚠️ (expect to receive a warning log) |
| true  | false  | `.medusa/server/public/admin`  |
| false  | true  | `.medusa/admin`  |
| false  | false  | none |

```diff
// medusa-config.ts

{
  // ...
  admin: {
-    outDir: 'some/path'
  }
}
```


cc @kasperkristensen @sradevski @olivermrbl

packages/medusa/src/loaders/admin.ts

@@ -3,6 +3,7 @@ import { AdminOptions, ConfigModule } from "@medusajs/framework/types"
 import { Express } from "express"
 import fs from "fs"
 import path from "path"
+import { ADMIN_RELATIVE_OUTPUT_DIR } from "../utils"
 
 type Options = {
   app: Express
@@ -10,10 +11,9 @@ type Options = {
   rootDirectory: string
 }
 
-type IntializedOptions = Required<
-  Pick<AdminOptions, "path" | "disable" | "outDir">
-> &
+type IntializedOptions = Required<Pick<AdminOptions, "path" | "disable">> &
   AdminOptions & {
+    outDir: string
     sources?: string[]
   }
 
@@ -39,6 +39,7 @@ export default async function adminLoader({
     disable: false,
     sources,
     ...admin,
+    outDir: path.join(rootDirectory, ADMIN_RELATIVE_OUTPUT_DIR),
   }
 
   if (adminOptions?.disable) {