Allow scim-kit to be used in Rails 6.0.x apps

[kamal]

Why is this needed?

I'm building a SCIM client which lives in a Rails 6.0 app. Currently, the gem has a strict dependency on activemodel 6.1 and above. I'm unable to install the gem.

What does this do?

The main change was to loosen the activemodel's version in the gemspec to include 6.0. To ensure it works, I added the appraisal gem as a development dependency and configured CI to run against 3 Rails versions: 6.0, 6.1, 7.0.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Verification Plan

• CI passes for all Rails versions via the appraisal gem

Discussion

• Should the different appraisals be defined as a matrix instead? This would blow up the combination of tests run in parallel due to the product of ruby version x activemodel version.
• I have unrelated fixes in here to update activemodel, rails-html-sanitizer and loofah to appease bundle audit.