Bump Ruby target version to 3.0

What does this MR do and why?

This is the oldest supported Ruby version on gitlab-org/gitlab.

Adjust required Ruby version accordingly. Ruby 2.6 reached end of life in April 2022.

Refs gitlab-org/gitlab!142681 (merged)

How to validate locally

ruby -v
ruby 3.2.3 (2024-01-18 revision 52bb2ac0a6) [x86_64-linux]

bundle exec ruby -rrubocop -e "p RuboCop::ConfigStore.new.for_file('.').target_ruby_version"
3.0