[release-3.6] Use RLock in Demoted method for read-only access to expiry

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #19432

/assign ahrtr

[k8s-ci-robot]

Hi @k8s-infra-cherrypick-robot. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ahrtr]

/ok-to-test

[ahrtr]

/test pull-etcd-coverage-report

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.95%. Comparing base (0e40f37) to head (d057562).
Report is 2 commits behind head on release-3.6.

Additional details and impacted files

Files with missing lines	Coverage Δ
server/lease/lease.go	100.00% <100.00%> (ø)

... and 20 files with indirect coverage changes

@@               Coverage Diff               @@
##           release-3.6   #19443      +/-   ##
===============================================
- Coverage        68.95%   68.95%   -0.01%     
===============================================
  Files              420      420              
  Lines            35753    35753              
===============================================
- Hits             24655    24653       -2     
- Misses            9679     9682       +3     
+ Partials          1419     1418       -1     

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[ahrtr]

#19447 should resolved the workflow failure.

cc @fuweid @ivanvc @jmhbnz

[fuweid]

/retest

[fuweid]

LGTM

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, fuweid, k8s-infra-cherrypick-robot

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