Removes the `gl_avatar_for_all_user_avatars` FF

What does this MR do and why?

It removes the FF gl_avatar_for_all_user_avatars which was enabled on production for about a month. We have some small hiccups #372609 (closed) and #373008 (closed) related to avatar rendering but they are being worked on in this milestone and I do not think should be blocking.

All the issues in the next 3 epics were closed which means that each instance of the new avatar implementation proved to work correctly or was fixed to work correctly.

&7610 (closed) &7731 (closed) &7767 (closed)

So that not to support 2 versions of avatars - old & new - it is about time to remove the FF.

In general this MR:

• Removes the FF
• Fixes the specs to pass with the new implementation
• Removes old avatar and old avatar link implementations and leaves only the new one, based on GlAvatar.
• Changes the default value for the avatar image size to 24 and cleans up all the places where the 24 size was set explicitly.

Closes #354887 (closed)

Screenshots or screen recordings

Here are just a couple of instances where the new avatar is used

Issues boards

How to set up and validate locally

MR discussion and apprvers

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #353477 (closed)