Add doc and test for set group avatar REST API

What does this MR do?

Allows a file to be submitted as avatar on API Update Group PUT groups/:id for Admin (It was already working during Create Group POST groups)

• Most of the code and tests were already there from the Create Group shared_examples, and the API doc already lists this param for updating group (PUT) as well as create (POST).

Updates Groups API doc under "Update group"

Closes #22453 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" "http://localhost:3000/api/v4/groups/67" --form "avatar=@/tmp/example.jpg"

Response:

{"id":67,"web_url":"http://127.0.0.1:3000/groups/h5bp/subgrphb5","name":"subgrphb5","path":"subgrphb5","description":"","visibility":"private","share_with_group_lock":false,"require_two_factor_authentication":false,"two_factor_grace_period":48,"project_creation_level":"developer","auto_devops_enabled":null,"subgroup_creation_level":"maintainer","emails_disabled":null,"mentions_disabled":null,"lfs_enabled":true,"default_branch_protection":2,"avatar_url":"http://127.0.0.1:3000/uploads/-/system/group/avatar/67/example.jpg","request_access_enabled":true,"full_name":"H5bp / subgrphb5","full_path":"h5bp/subgrphb5","created_at":"2020-12-29T16:42:24.321Z","parent_id":28,"ldap_cn":null,"ldap_access":null,"marked_for_deletion_on":null,"shared_with_groups":[],"projects":[],"shared_projects":[],"shared_runners_minutes_limit":null,"extra_shared_runners_minutes_limit":null,"prevent_forking_outside_group":false}

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #22453 (closed)