Update gems and drop support for Ruby versions < 3.1

[kenyonj]

Bumps the following gems:

• activesupport from 4.0 to 7.1.3.4
• minitest from 5.4 to 5.23.1
• nokogiri from 1.8.1 to 1.16.5
• nokogiri-diff from 0.2.0 to 0.3.0
• github-linguist from 7.1.3 to 7.30.0
• sanitize from 4.6.3 to 6.1.0
• html-pipeline from 1.0 to 2.14.3
  • Due to updates with html-pipeline, I'm also updating the fixtures to reflect the changes.

Drops support for:

• Ruby versions < 3.1.0