Fix MySQL db migration with default encoding/collation

[jedcunningham]

This allows the migrations to complete for MySQL db's without proper encoding/collation defaults.

I almost feel that we should be blocking migrations from running if they aren't set properly? Thoughts?

Related: #18939

[uranusjr]

I almost feel that we should be blocking migrations from running if they aren't set properly?

A check in upgradedb feels like a good idea to me.

[kaxil]

A check in upgradedb feels like a good idea to me.

Yup, a separate PR though

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[potiuk]

Agree. Checking it (incliding providing instructions on how to fix it) before migration is a very good idea (and agree with @kaxil it should be separate PR).

[potiuk]

Jeez. I hate MySQL.

[potiuk]

Good to go but. Should be rebased - we have the "migration" check to make sure that rebase happens for any migration changes (just to make sure we do not get into "double migration HEADs" state).