Upgrade Ruby to 2.7.5

[ChrisBAshton]

Upgrades Ruby to 2.7.5. See commits for more details.
This PR should be tested before merge, as per https://docs.publishing.service.gov.uk/manual/ruby.html#update-ruby-version-in-the-relevant-repos.

[KludgeKML]

• CI Passes. There's one deprecation warning, but it's something inside the govuk_publishing_components gem.
• Deployed to integration with no issues.
• Pages checked against production:
  • gov.uk root
  • https://www.gov.uk/government/get-involved
  • https://www.gov.uk/importing-vehicles-into-the-uk
  • https://www.gov.uk/government/publications/how-fair-are-the-uks-geospatial-assets/how-fair-are-our-national-geospatial-data-assets-assessment-of-the-uks-national-geospatial-data-html
  • https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/618167/government_dietary_recommendations.pdf
• No problems showing in Grafana or Sentry

[KludgeKML]

Looks good to me, contingent on the gem deprecation.