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/v1.8/pr-21808
Your branch is up to date with 'origin/v1.8'.

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/serve_repair.rs
	modified:   ledger/src/ancestor_iterator.rs
	modified:   ledger/src/blockstore_meta.rs
	modified:   local-cluster/tests/local_cluster.rs

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   core/src/repair_generic_traversal.rs
	both modified:   ledger-tool/src/main.rs
	both modified:   ledger/src/blockstore.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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[mergify]

automerge label removed due to a CI failure

[solana-grimes]

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

[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #22936 (ddf7a39) into v1.8 (430cdf6) will increase coverage by 0.0%.
The diff coverage is 99.2%.

@@           Coverage Diff            @@
##             v1.8   #22936    +/-   ##
========================================
  Coverage    81.6%    81.6%            
========================================
  Files         464      464            
  Lines      131755   131931   +176     
========================================
+ Hits       107623   107781   +158     
- Misses      24132    24150    +18     

[github-actions]

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