Support rails7 on travis ci

[tknzk]

Why?

Support Rails7
add CI for Rails7
#52

What?

modified Appraisals for rails 7
add gemfiles/rails_7.gemfile
modified travis.yml ( add rails7)
modified spec/dummy/config/initializers/assets.rb
fixed error below on rails7

>> BUNDLE_GEMFILE=/Users/xxxx/src/github.com/tknzk/scim_rails/gemfiles/rails_7.gemfile bundle exec rspec spec/

An error occurred while loading ./spec/controllers/scim_rails/scim_users_controller_spec.rb.
Failure/Error: Rails.application.config.assets.version = '1.0'

NoMethodError:
  undefined method `assets' for #<Rails::Application::Configuration:0x0000000152623a88>
  Did you mean?  asset_host
# ./spec/dummy/config/initializers/assets.rb:4:in `<top (required)>'
# ./spec/dummy/config/environment.rb:5:in `<top (required)>'
# ./spec/spec_helper.rb:3:in `require'
# ./spec/spec_helper.rb:3:in `<top (required)>'
# ./spec/controllers/scim_rails/scim_users_controller_spec.rb:1:in `require'
# ./spec/controllers/scim_rails/scim_users_controller_spec.rb:1:in `<top (required)>'

Caveats

N/A

Testing Notes

N/A

Alternatives Considered

N/A

Further Reading

N/A

Merge Instructions

Please DO NOT squash my commits when merging

[thedayisntgray]

Any blockers getting this in any time soon?

[thedayisntgray]

We should probably use a newer version of ruby that is not at end of life.
https://endoflife.date/ruby