Backport of "Preserve Geo verification retry fields across resyncs"

What does this MR do and why?

Backport of !248366 (merged) to 19.1.

Fixes #607991 (closed).

On a Geo secondary, verification_retry_count could never exceed 1. A verification failure calls failed, which schedules a resync. The resync's :start transition disables verification, and after_synced then moves the registry from verification_disabled to verification_pending, which cleared verification_retry_count, verification_retry_at and checksum_mismatch. So the progressive backoff of syncs-due-to-verification-failures never engaged: a registry with a genuine checksum mismatch kept resyncing at full speed instead of backing off.

The fix includes verification_disabled in the transition that preserves those fields, alongside verification_failed.

Cherry-picked cleanly. No conflicts, and no changes from the version merged into master (verified with git range-diff).

The regression came in with 08492a70, which is in 19.1 and later but not in 19.0, so 19.0 does not need this backport.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

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