Remove support for Ruby 3.0

Ruby 3.0 is not supported by the ruby-core team any longer and does not receive any fixes. No further patch release will be released. 

[Ruby 3.0 EOL status](https://www.ruby-lang.org/en/downloads/branches/).

[GOV.UK's Ruby version compatibility docs](https://docs.publishing.service.gov.uk/manual/publishing-a-ruby-gem.html#ruby-version-compatibility)

This PR will be reviewed and merged by the Platform Security and Reliability team. Any questions or concerns, please reach out in our channel: #govuk-platform-security-reliability-team.

[Trello card](https://trello.com/c/StwaSbSh/3549-drop-support-for-ruby-30-in-our-gems-2)

.ruby-version

@@ -1 +1 @@
-3.0.5
+3.1.5