Test against Rails 8.0, drop support for Rails 5.2

[mak-dunkelziffer]

This gem has some version switches:

Rails.version.to_i < 3
Rails.version.to_i < 4
Rails.version.to_i < 5

We don't test against any of those versions anymore. We also want to phase out support for older versions of Ruby and Rails. Should I remove them?

Also, should I increase version constraints in the gemspec?

s.add_dependency('memoized', '>=1.0.2')
s.add_dependency('activerecord', '>= 3.2')
s.add_dependency('activesupport', '>= 3.2')
s.add_dependency('railties', '>= 3.2')
s.add_dependency('edge_rider', '>= 0.3.0')

[dastra-mak]

LGTM