fix: Add m.room.avatar to the required state

[jmartinesp]

This works around the issue with removed avatars not being explicitly flagged by sliding sync until element-hq/synapse#18598 is fixed.

This is a workaround for #5285.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[bnjbvr]

I smell incoming test failures, but LGTM once they're addressed!

[jmartinesp]

I smell incoming test failures, but LGTM once they're addressed!

Agh, I forgot to run nextest. Let's see how many need to be fixed 🤞 .

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.19%. Comparing base (836c643) to head (f7b1480).
Report is 6 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5293      +/-   ##
==========================================
- Coverage   90.19%   90.19%   -0.01%     
==========================================
  Files         334      334              
  Lines      105155   105149       -6     
  Branches   105155   105149       -6     
==========================================
- Hits        94847    94840       -7     
  Misses       6245     6245              
- Partials     4063     4064       +1     

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