Add update avatar on user api

What does this MR do and why?

Add Update avatar on user api

Endpoint: user/avatar

This MR allows users to customize avatars for group and project access token bots.

Connected to : Allow avatar customization for group or project... (#356868 - closed)

How to set up and validate locally

curl --request PUT \
  --url http://gdk.test:3000/api/v4/user/avatar \
  --header 'Authorization: Bearer <PROJECT_OR_GROUP_BOT_ACCESS_TOKEN' \
  --header 'Content-Type: multipart/form-data' \
  --form 'avatar=@<PATH_TO_FILE>'

doc/api/users.md

@@ -2554,9 +2554,9 @@ Upload an avatar to current user.
 PUT /user/avatar
 ```
 
-| Attribute | Type              | Required | Description |
-|-----------|-------------------|----------|-------------|
-| `avatar`  | string            | Yes      | The file to be uploaded. |
+| Attribute | Type              | Required | Description                                                                                                 |
+|-----------|-------------------|----------|-------------------------------------------------------------------------------------------------------------|
+| `avatar`  | string            | Yes      | The file to be uploaded. The ideal image size is 192 x 192 pixels. The maximum file size allowed is 200KiB. |
 
 To upload an avatar from your file system, use the `--form` argument. This causes
 cURL to post data using the header `Content-Type: multipart/form-data`. The
@@ -2567,54 +2567,19 @@ Example request:
 
 ```shell
 curl --request PUT --header "Bearer: <your_access_token>" \
-     --form "avatar=@avatar.png" "https://gitlab.example.com/api/v4/user/avatar"
+     --form "avatar=@avatar.png" \
+     --url "https://gitlab.example.com/api/v4/user/avatar"
 ```
 
 Returned object:
 
+Returns `400 Bad Request` for file sizes greater than 200 KiB.
+
+If successful, returns [`200`](rest/index.md#status-codes) and the following
+response attributes:
+
 ```json
 {
-  "id": 76,
-  "username": "group_33_bot_204e254b386e6445323593946ec746d5",
-  "name": "****",
-  "state": "active",
-  "locked": false,
-  "avatar_url": "http://gdk.test:3000/uploads/-/system/user/avatar/76/.dk.png",
-  "web_url": "http://gdk.test:3000/group_33_bot_204e254b386e6445323593946ec746d5",
-  "created_at": "2024-04-14T02:24:21.475Z",
-  "bio": "",
-  "location": "",
-  "public_email": null,
-  "skype": "",
-  "linkedin": "",
-  "twitter": "",
-  "discord": "",
-  "website_url": "",
-  "organization": "",
-  "job_title": "",
-  "pronouns": null,
-  "bot": true,
-  "work_information": null,
-  "followers": 0,
-  "following": 0,
-  "local_time": null,
-  "last_sign_in_at": null,
-  "confirmed_at": "2024-04-14T02:24:21.178Z",
-  "last_activity_on": "2024-04-14",
-  "email": "group_33_bot_204e254b386e6445323593946ec746d5@noreply.gdk.test",
-  "theme_id": 3,
-  "color_scheme_id": 1,
-  "projects_limit": 100000,
-  "current_sign_in_at": null,
-  "identities": [],
-  "can_create_group": true,
-  "can_create_project": true,
-  "two_factor_enabled": false,
-  "external": false,
-  "private_profile": false,
-  "commit_email": "group_33_bot_204e254b386e6445323593946ec746d5@noreply.gdk.test",
-  "shared_runners_minutes_limit": null,
-  "extra_shared_runners_minutes_limit": null,
-  "scim_identities": []
+  "avatar_url": "http://gdk.test:3000/uploads/-/system/user/avatar/76/avatar.png",
 }
 ```