Update CI matrix & Devise version constraint

[moritzhoeppner]

Hi @erdostom, I updated the version matrix for the CI pipeline and allowed Devise 5 in the gemspec:

1. I removed outdated versions of dependencies from the CI pipeline. Specifically:

• Rails < 7.2 doesn't get security fixes, so I don't think we have to support it.
• Rails 7.2.0 requires at least Ruby 3.1.0, so we don't need tests for older Ruby versions.
• Mongoid 7.5 requires ActiveModel < 7.1, so we don't need to test against it.

2. By increasing the patch version of Mongoid 8.1 and 9.0, we get support for Rails 8.0 and can test against it.
3. I added Rails 8.1. As of now, Mongoid doesn't support it.
4. I added Devise main. I think this justifies relaxing the version constraint for Devise from "~> 4.0" to ">= 4.0". [*] This solves Support for Devise v5? #23.

[*] Actually, I don't think that 4.0 is the most precise lower bound for the Devise version. Devise requires Rails < 6.0 until version 4.6.2 - so I think we could change the version constraint for Devise to ">= 4.7.0". But it probably doesn't matter since older versions can't be used anyway.

[erdostom]

Thank you Moritz!