[release-3.6] Move interface AlarmBackend from server/etcdserver/api/v3alarm to server/storage/schema to fix the unexported-return lint error

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #19342

/assign ivanvc

[k8s-ci-robot]

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

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[ivanvc]

/ok-to-test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.91%. Comparing base (fd47d1b) to head (1fd8887).
Report is 2 commits behind head on release-3.6.

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/api/v3alarm/alarms.go	84.21% <100.00%> (ø)
server/storage/schema/alarm.go	86.66% <100.00%> (ø)

... and 20 files with indirect coverage changes

@@             Coverage Diff              @@
##           release-3.6   #19455   +/-   ##
============================================
  Coverage        68.91%   68.91%           
============================================
  Files              420      420           
  Lines            35753    35753           
============================================
+ Hits             24638    24640    +2     
- Misses            9687     9689    +2     
+ Partials          1428     1424    -4     

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[ivanvc]

/retest

[ivanvc]

Backport LGTM. Defer the decision if we want it in v3.6.0 to @ahrt. However, as I mentioned if we don't do it, we'll need to deprecate in main.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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