Update changelog with v3.5.19 v3.6.0-rc.2 release dates

[ivanvc]

Part of #19494, #19489.

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 68.92%. Comparing base (f0a1ec5) to head (5dea90c).
Report is 3 commits behind head on main.

Additional details and impacted files

see 21 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19543      +/-   ##
==========================================
+ Coverage   68.90%   68.92%   +0.01%     
==========================================
  Files         421      421              
  Lines       35901    35901              
==========================================
+ Hits        24738    24744       +6     
+ Misses       9740     9737       -3     
+ Partials     1423     1420       -3     

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[jmhbnz]

LGTM - Thanks @ivanvc

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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