Default GDK to use PostgreSQL 12.6

What does this Merge Request do and why?

This MR changes the default and active version of PostgreSQL to 12.6.

When a gdk update is run, the following will be shown:

$ gdk update
--snip--
❌️ ERROR: PostgreSQL data directory is version 11 and must be upgraded to version 12.6 before GDK can be updated.

Run 'support/upgrade-postgresql' to back up and upgrade the PostgreSQL data directory.

make: *** [preflight-update-checks] Error 1

$ support/upgrade-postgresql
ℹ️  Available PostgreSQL versions: {11=>"/Users/ash/.asdf/installs/postgres/11.9/bin", 12=>"/Users/ash/.asdf/installs/postgres/12.6/bin"}
ℹ️  Initializing '/Users/ash/src/gitlab/gitlab-development-kit/postgresql/data.12.1619752346' for PostgreSQL 12..
ℹ️  Renaming /Users/ash/src/gitlab/gitlab-development-kit/postgresql/data to /Users/ash/src/gitlab/gitlab-development-kit/postgresql/data.11.1619752347
ℹ️  Upgrading '/Users/ash/src/gitlab/gitlab-development-kit/postgresql/data.11.1619752347' (PostgreSQL 11) to '/Users/ash/src/gitlab/gitlab-development-kit/postgresql/data.12.1619752346' PostgreSQL 12..
ℹ️  Promoting newly-creating database from '/Users/ash/src/gitlab/gitlab-development-kit/postgresql/data.12.1619752346' to '/Users/ash/src/gitlab/gitlab-development-kit/postgresql/data'
ℹ️  Running 'gdk reconfigure'..
✅️ Upgraded '/Users/ash/src/gitlab/gitlab-development-kit/postgresql/data' from PostgreSQL 11 to 12.

$ gdk update
--snip--
> Updated as of 2021-04-30 13:18:18. Took 260 second(s).

😀

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1166 (closed)