Fix jsonb concatenation in down migration

[matthew-white]

While working on #1090, I took a look at how previous migrations had updated verbs. I noticed the use of the + operator in one migration, but later I realized that that's not actually a valid operator for the verbs column, which is jsonb. We use the || operator in other migrations, so I went ahead and replaced the + with ||. Clearly, the + wasn't actually causing any issues: we must never have needed to run the down migration. But it was a little confusing when I was trying to find prior related work and came across the operator.

What has been done to verify that this works as intended?

I didn't do anything to verify the change, since it seems clear that we've never needed to run the down migration. I just wanted to make the code a little clearer and more consistent with other migrations.

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• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced