Do not test Rails 7.0 against Ruby 3.4 dev

Stop testing Rails 7.0 against ruby-head, as 7.0 will now only receive
security fixes.
Compatibility with future Ruby versions will be managed in newer
Rails versions.

Ref: rails/rails#50546

.github/workflows/ruby.yml

@@ -40,10 +40,6 @@ jobs:
             gemfile: rails_edge
             postgres-version: '16'
             channel: 'experimental'
-          - ruby-version: 'head'
-            gemfile: rails_7.0
-            postgres-version: '16'
-            channel: 'experimental'
           - ruby-version: 'head'
             gemfile: rails_7.1
             postgres-version: '16'