Drop support for end-of-life Ruby versions + update dependencies.

[sengi]

• Drop support for Ruby 2.7 and 3.0 per our version policy.
• Update (theoretically) vulnerable npm packages (npm audit fix).
• Update rubygems dependencies as far as compatible.
• Update GitHub Actions dependencies.
• Declare some missing transitive dependencies for forward compatibility with Ruby 3.4.

Someone with repo admin (or alphagov org admin) will need to update the repo's branch protection rules to fix the list of required checks.

[heathd]

I've removed the requirement for tests on ruby 2.7

will try to do a review in the next day or two (probably by just including the gem in another thing and making sure it works)

[heathd]

I have tested against a local tech docs site using ruby 3.2.3. Everything seems to function correctly.

[sengi]

Thanks @heathd!