Support Rails 8.0

[zhuravel]

Issue #, if available: —

Description of changes:

• Add support for Rails 8.0
• Add Ruby 3.3, Rails 8.0, PostgreSQL 17, MySQL 9.1 to tests
• Remove MySQL 5.5 from tests as its support ended in 2018 and it’s not supported by Rails 8.0
• Upgrade versions of GitHub actions to fix deprecation warnings

Important

The only test failing is this one:

Please make sure test coverage is 100%. Currently: 100% (line), 97.14% (branch).

Not sure why branch coverage decreased, any suggestions are welcome.

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[dmytro-savochkin]

Hey @zhuravel, thanks for the great PR! Appreciate it!

I have two comments:

1. Could we actually keep Gemfile kinda empty as it was before? At least to me, it feels more obvious this way when every dependency for a gem is defined inside .gemspec file. And this change is not strictly necessary for anything in this PR.
2. I'd really like to get down to what caused the branch test coverage decrease but I don't have time to research it right now, so I will try to look at it a bit later the next week.

[dmytro-savochkin]

Okay, I've done some debugging. Apparently the problem is with collating Simplecov does to calculate branch test coverage across different ruby and rails versions. We have to do it because of this line.
However I don't understand what exactly causes it. And as it turns out if I push the exact same version of code that was green a year ago, now it's red. So it has nothing to do with the changes in this PR. I think I am gonna just merge it and decrease the coverage threshold.