uses Option<Slot> for SlotMeta.parent_slot (backport #21808)

[mergify]

This is an automatic backport of pull request #21808 done by Mergify.
Cherry-pick of 8d980f0 has failed:

On branch mergify/bp/v1.9/pr-21808
Your branch is up to date with 'origin/v1.9'.

You are currently cherry-picking commit 8d980f07b.
  (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:   core/src/repair_generic_traversal.rs
	modified:   core/src/serve_repair.rs
	modified:   ledger/src/ancestor_iterator.rs
	modified:   ledger/src/blockstore.rs
	modified:   ledger/src/blockstore_meta.rs
	modified:   local-cluster/tests/local_cluster.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   ledger-tool/src/main.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 #21899 (717c99f) into v1.9 (a9d4728) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             v1.9   #21899     +/-   ##
=========================================
- Coverage    81.3%    81.3%   -0.1%     
=========================================
  Files         515      515             
  Lines      143700   143696      -4     
=========================================
- Hits       116845   116826     -19     
- Misses      26855    26870     +15     

[github-actions]

This PR has been automatically locked since there has not been any activity in past 14 days after it was merged.