Add banner to announce end of support for legacy validator

[effigies]

Closes #2192.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.70%. Comparing base (1ea4a93) to head (d4f8237).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2236   +/-   ##
=======================================
  Coverage   85.70%   85.70%           
=======================================
  Files          91       91           
  Lines        3784     3784           
  Branches     1221     1221           
=======================================
  Hits         3243     3243           
  Misses        455      455           
  Partials       86       86           

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[effigies]

@Remi-Gau Sorry for being so slow at this one. Would you mind having a look? Suggestions for text or styling welcome, but I don't think we need much here.

This is in prep of archiving the repo.