fix(db) blue/green migration for route regex path

[StarlightIbuki]

The migration steps should happen in the up phase, not the teardown phase.

Notice this PR needs cooperation from 2.8.x.x.

fix FT-3293

[jschmid1]

@suika-kong could you please adapt the commit message so that it is compliant to https://www.conventionalcommits.org/en/v1.0.0/

[bungle]

@suika-kong could you please adapt the commit message so that it is compliant to https://www.conventionalcommits.org/en/v1.0.0/

I think this is our format:
https://github.com/Kong/kong/blob/master/CONTRIBUTING.md#commit-message-format

[bungle]

If I remember correctly, the original idea of migrations was to do:

1. schema changes in up (non-destructive)
2. data changes in finish

We have done data changes in up before, and sometimes they work, and sometimes they cause huge issues. Especially if there are both schema changes and data changes. This is especially problematic in Cassandra clusters, if I remember correctly.

That said I know that some migrations are impossible to do that way. Which usually means they need to be done in parts, in several minor releases, or apply code changes. This is cumbersome for sure too.

[yzyyi]

Is this one still target 3.0? Trying to decide if needed cherrypick

[StarlightIbuki]

We decided to only support blue/green migration from 2.8.1.5 where we support both ~ and auto-detection(but users are not allowed to create ~ regex path).