Remove ruby 2.7 and use 3.0

[sjha4]

Rubocop started failing with this:

Installing Bundler
> bundle install
/opt/hostedtoolcache/Ruby/2.7.8/x64/bin/bundle config --local path /home/runner/work/smart_proxy_container_gateway/smart_proxy_container_gateway/vendor/bundle
/opt/hostedtoolcache/Ruby/2.7.8/x64/bin/bundle lock
Fetching https://github.com/theforeman/smart-proxy.git
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Could not find compatible versions

Because every version of smart_proxy depends on Ruby >= 3.0
  and Gemfile depends on smart_proxy >= 0,
  Ruby >= 3.0 is required.
So, because current Ruby version is = 2.7.8,
  version solving has failed.
Error: The process '/opt/hostedtoolcache/Ruby/2.7.8/x64/bin/bundle' failed with exit code 6

This repo is using 2.7 where dependencies are at 3.0.

[sjha4]

@evgeni : Does this look right?