[keyserver] Update updateUserAvatarResponder to return CreateUpdatesResult for new clients

Summary:
For new clients (>215), we return CreateUpdatesResult instead of ClientAvatar.

Context can be found:

ENG-3808 : updateUserAvatar should create UPDATE_USER and UPDATE_CURRENT_USER updates

https://linear.app/comm/issue/ENG-3809/add-code-to-reduceuserinfos-to-handle-user-setting-their-own-avatar

Test Plan:
Log CreateUpdatesResult on keyserver and ensure that it's as expected:

Will be tested implicitly by subsequent diff which includes necessary change on client (w/ codeVersion bumped to 215 for testing)

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7699