Update minimum Ruby to 2.7

[unleashed]

This PR includes changes to CI so we now only use Ruby 2.7:

• The CI image is now based on CentOS 8 and includes just Ruby 2.7.
• The tested versions of Redis and PostgreSQL have been updated to the versions shipping on RHEL 8.
• The pg gem needed an upgrade and a minimal code change.

Ruby 3.0 requires further changes in dependencies that check for 2.X versions of Ruby.

Pending:

• [x] ~Update the release image.~ Already done by David.

As a side note:

The CI image cannot be converted to CentOS Stream because no such container image is published, and UBI8 which is touted as the ideal RHEL for CI container image just... won't... install... bison and similarly ultra-basic packages for development. :/

This is part of the work needed for #303.

[unleashed]

bors r+

[bors[bot]]

Build succeeded:

• ci/circleci: build