Update GitHub Actions runner to ubuntu-24.04

[snaka]

Change to ubuntu-24.04 since ubuntu-20.04 is not supported in the current GitHub Acions.

Also, Rails 8.0 doesn't support Ruby 3.1.

See also:

• Ubuntu 20 Deprecation changes actions/runner-images#11748

[snaka]

@nashby
If you are the maintainer of this repository, I would appreciate it if you could take a moment to review this PR.

Thank you.

[snaka]

[note]

The test that is failing in this test (the one about Rails8+) has been fixed in the following PR.
Those failing tests can be ignored for now and can be merged in.

• Fix Type#cast to support Rails 8.0 bulk operations #469

I confirmed that tests pass by merging this PR ( #466 ) with the related PR ( #469 ).

• [wip] Temp fix type cast for rails80 snaka/enumerize#3

[snaka]

@nashby

It is helpful if you could merge this PR first, considering the following points.

The test that is failing in this test (the one about Rails8+) has been fixed in the following PR.
Those failing tests can be ignored for now and can be merged in.

• Fix Type#cast to support Rails 8.0 bulk operations #469

If you feel that the current situation where the test is failing is undesirable, I can also include the above PR in this PR.
If you would like me to do so, please comment.

Thank you.

[nashby]

@snaka thanks!