Drop TargetRubyVersion from RuboCop config

We initially set the targeting for RuboCop here:
https://github.com/navapbc/template-application-rails/pull/27/files

But RuboCop should automatically pickup the version from
`.ruby-version` (or the `Gemfile.lock`)[1], which seems to be working.
So drop the redundancy, which will make language updates a bit more
automatable.

[1]: https://docs.rubocop.org/rubocop/configuration.html#setting-the-target-ruby-version

Author: doshitan

template/{{app_name}}/.rubocop.yml

@@ -3,5 +3,3 @@ require:
 inherit_gem:
   pundit: config/rubocop-rspec.yml
   rubocop-rails-omakase: rubocop.yml
-AllCops:
-  TargetRubyVersion: 3.4.2