Fix "No such column: geo_nodes.verification_max_capacity"

What does this MR do and why?

Fix "Upgrade to 18.0: No such column: geo_nodes.verification_max_capacity".

Unexpectedly, some installations have a geo_nodes table missing 1 or 2 columns.

Why modify the existing migration rather than add a new one?

I initially attempted to add a new migration which adds the columns if they don't exist, but even though I specifically chose a migration timestamp earlier than !184473 (merged), when I rolled back that migration and migrated again, it ran out of order, and so the migration that changes the defaults failed.

So in order to guarantee that columns are not attempted to be modified when they don't exist, I changed approach to modifying the existing migration.

References

Issue: #543146 (closed)

Backport to 18.0: !192175 (merged)

How to set up and validate locally

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