Handle already discarded packets in discard_excess_packets (backport #22594)

[mergify]

This is an automatic backport of pull request #22594 done by Mergify.
Cherry-pick of 38b02bb has failed:

On branch mergify/bp/v1.8/pr-22594
Your branch is up to date with 'origin/v1.8'.

You are currently cherry-picking commit 38b02bbcc.
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
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	both modified:   core/src/sigverify_stage.rs

no changes added to commit (use "git add" and/or "git commit -a")

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.

[mergify]

automerge label removed due to a CI failure

[mergify]

automerge label removed due to a CI failure

[mergify]

automerge label removed due to a CI failure

[github-actions]

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