Add rails 7 testing, drop ruby 2.6

Ruby 2.6 is EOL and not supported by rails 7.  If we really need it, can
test ruby 2.6 against rails < 7.

.github/workflows/ci.yaml

@@ -12,12 +12,12 @@ jobs:
     strategy:
       matrix:
         ruby-version:
-        - '2.6'
         - '2.7'
         - '3.0'
         rails-version:
         - '6.0'
         - '6.1'
+        - '7.0'
     env:
       TEST_RAILS_VERSION: ${{ matrix.rails-version }}
       CC_TEST_REPORTER_ID: ${{ secrets.CC_TEST_REPORTER_ID }}

Gemfile

@@ -3,9 +3,7 @@ source 'https://rubygems.org'
 # Specify your gem's dependencies in manageiq-messaging.gemspec
 gemspec
 
-case ENV['TEST_RAILS_VERSION']
-when "6.0"
-  gem "activesupport", "~>6.0.4"
-when "6.1"
-  gem "activesupport", "~>6.1.4"
-end
+version = ENV.fetch('TEST_RAILS_VERSION', '6.1')
+raise "Unsupported version of Rails: #{version}" unless %w[6.0 6.1 7.0].include?(version)
+
+gem "activesupport", "~>#{version}", "< #{version.to_f + 0.1}"