added deprecation message above SiteIcon

[thebinij]

Explanation

• Fixes Deprecate SiteIcon in favour of AvatarFavicon  #18715

Manual Testing Steps

Pre-merge author checklist

• I've clearly explained:
  • What problem this PR is solving
  • How this problem was solved
  • How reviewers can test my changes
• Sufficient automated test coverage has been added

Pre-merge reviewer checklist

• Manual testing (e.g. pull and build branch, run in browser, test code being changed)
• PR is linked to the appropriate GitHub issue
• IF this PR fixes a bug in the release milestone, add this PR to the release milestone

If further QA is required (e.g. new feature, complex testing steps, large refactor), add the Extension QA Board label.

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[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[thebinij]

Hi @georgewrmarshall, I was unable to create an issue that is not "type-bug". Please review this PR. Thank you.

[codecov]

Codecov Report

Merging #19256 (c4d6e2b) into develop (d9c48cc) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop   #19256      +/-   ##
===========================================
- Coverage    70.36%   70.35%   -0.01%     
===========================================
  Files          961      961              
  Lines        36845    36845              
  Branches      9588     9588              
===========================================
- Hits         25924    25922       -2     
- Misses       10921    10923       +2     

Impacted Files	Coverage Δ
ui/components/ui/site-icon/site-icon.js	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

[georgewrmarshall]

Excellent! Thanks for your contribution @thebinij! I made a small update.