CHANGELOG-3.6: Add note from #19404

[ivanvc]

Add a deprecation note from #19404.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.90%. Comparing base (7073c41) to head (8af64f8).
Report is 7 commits behind head on main.

Additional details and impacted files

see 15 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19407      +/-   ##
==========================================
- Coverage   68.93%   68.90%   -0.03%     
==========================================
  Files         420      420              
  Lines       35757    35757              
==========================================
- Hits        24649    24639      -10     
- Misses       9683     9696      +13     
+ Partials     1425     1422       -3     

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[k8s-ci-robot]

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