CHANGELOG: update 3.6 changelog with DowngradeInfo

[fuweid]

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 68.86%. Comparing base (2de17bd) to head (a081904).
Report is 7 commits behind head on main.

Additional details and impacted files

see 25 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19474      +/-   ##
==========================================
+ Coverage   68.84%   68.86%   +0.01%     
==========================================
  Files         420      420              
  Lines       35762    35762              
==========================================
+ Hits        24622    24626       +4     
+ Misses       9714     9708       -6     
- Partials     1426     1428       +2     

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[ahrtr]

/retest

[ivanvc]

LGTM. Thanks, @fuweid.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, fuweid, ivanvc

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[ahrtr]

Minor comment: add a < hr > at the end. Leave it to @ivanvc fix it when updating the release date.

[ivanvc]

Noted.