message: chore: Update PR template for DB migrations process
diff: <!--- HINT: Include "Fixes #nnn" if you are fixing an existing issue -->
  - [ ] Has associated issue:
  - [ ] Changes UI
  -- [ ] Requires DB Migration.
  -- [ ] Confirm DB Migration upgrade and downgrade tested.
  +- [ ] Includes DB Migration (follow approval process in [SIP-59](https://github.com/apache/superset/issues/13351))
  + - [ ] Migration is atomic, supports rollback & is backwards-compatible
  + - [ ] Confirm DB migration upgrade and downgrade tested
  + - [ ] Runtime estimates and downtime expectations provided
  - [ ] Introduces new feature or API
  - [ ] Removes existing feature or API
  