Switch CDash DB backend from MySQL to Postgres

[williamjallen]

Kitware is planning to drop support for MySQL as a CDash database backend to make better use of Postgres-specific features. This issue is a placeholder for the task of migrating our existing Aurora MySQL instance to Aurora Postgres.

[williamjallen]

@jwnimmer-tri Re: the "reset the CDash database" idea, it looks like Drake's CDash instance is configured to retain 5 months of data.

[jwnimmer-tri]

I imagine we're not the only user of CDash that needs to migrate from Aurora to Aurora. Can you explain in the issue here what the challenge is? I know we talked about it in the meeting yesterday but it didn't make it into the notes.