[keyserver] Deprecate MySQL support
Summary:
This diff causes the app to crash if dbType is missing (as it previously defaulted to 'mysql5.7'), or if it is present and set to 'mysql5.7'.
Depends on D4832
Test Plan:
- Flow
- I ran keyserver locally and made sure things generally still worked
Reviewers: tomek, atul
Reviewed By: tomek
Subscribers: adrian, abosh
Differential Revision: https://phab.comm.dev/D4833