Geo: Reduce batch size of verification state backfill

What does this MR do and why?

Reduce batch size of verification state backfill to reduce resource usage, especially in combination with !136275 (merged). But neither MR is dependent on the other.

Partial solution for #429242 (closed)

Related to #430913 (closed)

• !136356 (merged) is in 16.6 and
  • is likely to make 16.5
  • unsure about 16.4
• !136394 (merged) is in 16.7 and
  • is likely to make 16.6.1/2
  • unsure about 16.6.0
  • unsure about 16.5
  • unlikely to make 16.4
• !136356 (merged) (this MR) is really easy and safe to backport, so we can probably get it in as far back as !136356 (merged)

I think batch size > 1000 is actually not an issue in 16.7+ because of !136394 (merged). My aim with this MR is more to mitigate #429242 (closed) as far back as possible with the least risk.

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.