Support Rails 8.0 routes

[zealot128]

When using Rails 8 (main branch checkout), there are a bunch of errors (kwargs) and deprecation warnings, because Rails deprecates Routes.rb post :something => 'action', arrow functions because of performance optim PR

Here is the diff, that fixes it for me.

As Rails8 is not released, it would be best, to wait to merge this until beta-releases, in case they change anything about this feature, or new deprecations occur.

In the meantime, other people can use my branch:

 gem 'trestle', git: 'https://github.com/zealot128-os/trestle.git', branch: 'rails8'

[coveralls]

coverage: 90.597% (-0.09%) from 90.682%
when pulling ce6396b on zealot128-os:rails8
into a18e54f on TrestleAdmin:main.

[spohlenz]

Thank you for this. I've added in Rails 8.x Gemfiles in #471 to continue tracking any breaking changes.

Please review my code comment when you get a chance and then I'll get this merged in.

[spohlenz]

Thanks again. I've added #472 to deal with the remaining deprecation warnings.