Migrate db.String to db.Text to support MySQL

Summary:
To support MySQL, we migrate incompatible column types. For example, it does not support a bare db.String, as it is not fixed length.
Here, we migrate all uses of db.String to db.Text to accommodate for this.

Co-authored-by: Rex Ledesma <rex@elementl.com>

Test Plan:

• bk
• Added new schema check to prevent addition of new db.String fields.
• Tested incorrect db.String column and new buildkite check fails for it: https://buildkite.com/dagster/dagster-diffs/builds/13894

Reviewers: nate, prha

Reviewed By: prha

Differential Revision: https://dagster.phacility.com/D5805