stake: Advance credits_observed on activation epoch (backport #19309)

[mergify]

This is an automatic backport of pull request #19309 done by Mergify.
Cherry-pick of 2c3bded has failed:

On branch mergify/bp/v1.7/pr-19309
Your branch is up to date with 'origin/v1.7'.

You are currently cherry-picking commit 2c3bdedea.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   program-test/tests/warp.rs
	modified:   programs/stake/src/stake_state.rs
	modified:   sdk/src/feature_set.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   runtime/src/bank.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[solana-grimes]

😱 New commits were pushed while the automerge label was present.

[codecov]

Codecov Report

Merging #19626 (36d15fd) into v1.7 (105a891) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##             v1.7   #19626   +/-   ##
=======================================
  Coverage    82.2%    82.2%           
=======================================
  Files         437      437           
  Lines      123934   123953   +19     
=======================================
+ Hits       101926   101968   +42     
+ Misses      22008    21985   -23