Align heroku database versions

[Pomax]

We're currently using four different postgresql versions:

• production = 9.6.2
• staging = 10.15
• review apps = 12.5
• local dev = 10.12

Let's get these all on the same version, or we're going to be chasing database ghosts, where one system's failing while everything else shows no indication of failure.

[tomusher]

As a first step for this I've added a new Postgres 12.5 addon to foundation-mofostaging-net and donate-wagtail-staging, copied across the data and made them the primary database. The old versions remain in case we need to rollback.

@pomax could you check staging on these sites to make sure everything works as you expect?

[Pomax]

very nice, as far as I can tell this is working the same as before.

[tomusher]

I've removed the old 9.6 staging databases and done the same upgrades to the two production sites, leaving the old databases in place for now.