Add Rails 8 support

[teeparham]

This PR is #419, plus two small commits:

• Remove ruby 3.1 from the test matrix, since rails 8 requires ruby 3.2
• Update the .gitignore file.

[teeparham]

Failures seem to be related to rails/rails#53904

[januszm]

I was about to suggest this test matri change to the previous PR

Bundler found conflicting requirements for the Ruby version:
  In Gemfile:
    activerecord-postgis-adapter was resolved to 10.0.0, which depends on
      Ruby (>= 3.2.0)

I think there's absolutely nothing wrong in requiring Ruby 3.2+ for this release. Applications, projecs, people who need to stick to older Ruby versions will most likely also need to stick to older Rails/ActiveRecord releases and can use older versions of this gem activerecord-postgis-adapter. Here, we can simply drop support for Ruby 3.1 with the next release. I've tested that PR in production for weeks and it works without issues.

[keithdoggett]

Thanks @teeparham. This looks good! The issue you linked seems to be for rails 7.2. Do you know if it's going to be merged into the next release of 8.0 too? I tried looking around release notes and commit history but didn't see anything.

[keithdoggett]

@januszm agree. Rails 8 requires 3.2+ so we should match that.