add tests on ruby 3.1, 3.2, rails 7.0, drop support for rails 5.1

[razumau]

Adds tests on Ruby 3.1 and 3.2 and Rails 7.0. Drops support for Rails 5.1.

Since Rails 7, datetime columns have precision 6 by default. I set the same value explicitly in the test database and switched time comparisons in several tests to rounding timestamps with .to_i.

[bquorning]

Why do we set the precision to 6 digits in the schema and then use 0 digits when we compare timestamps?

[razumau]

These were separate changes: I set precision to 6 for consistency across tests, and then decided that to_i’s precision is good enough for these tests. Do you think we should test these more precisely?

[bquorning]

The to_i conversion should not be necessary, especially inside a Timecop.freeze block. If the to_i is necessary for making some of the tests pass, I suspect those tests fail once in a while when rounding 3.999997 to 3 and 4.000002 to 4.

[razumau]

Reworked it to use Time#floor. Maybe there’s a better way to do this? The problem is that while in the database we now only have precision 6, Time.now still has additional nanoseconds, so we have to either cut them off or make a comparison less strict.