refactor renovate config

General MR

Summary

1. Fix renovate changelog generation.
2. Use renovate's helm-values built-in manager rather than custom regex managers.
3. Explicitly specify the registry for each mysql subchart image to allow renovate to detect updates for them.

Relevant logs/screenshots

update-everything

library-subcharts-only

app-image

mysql-image

chart-only

Linked Issue

Closes #246 (closed).

Upgrade Notices

N/A