Update CI to run against Ruby 3.3, drop Ruby 2.6

[owst]

google-protobuf supports Ruby 3.3 in platform-specific versions of 3.25.3

Some discussion in this thread: protocolbuffers/protobuf#15336 (comment)

[lfittl]

@owst Thanks for the contribution!

Looks like we do also need to bump up protobuf (at least a bit) to get this to work? (see failing CI)

[owst]

Oops, thanks for the ping @lfittl - I think my eyes read the gemspec requirement as already being >= 3.25.3 rather than >= 3.22.3 🤦

[owst]

Hmm, it looks like google-protobuf-3.25.3 doesn't support Ruby 2.6 (which is EOL) - I guess it's time to drop support for 2.6 in pg_query.

N.B. 2.7 is also EOL I wonder if by dropping support rubocop could be updated from its ancient version so it doesn't need to be skipped on PRs