Geo: Reduce batch size of verification state backfill

What does this MR do and why?

Describe in detail what merge request is being backported and why

Mitigates memory spikes by reducing batch size of a job, in combination with !136774 (merged).

Partial solution to #429242 (closed). (Depending on the environment, this will mitigate the problem completely enough for some folks.)

Related to #430913 (closed)

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).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

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• Refer to the patch release runbook for engineers and maintainers for guidance.
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