Minimal support for Ruby 3

[PeterJCLaw]

This isn't officially supported by Jekyll and is a bit hacky, so I'm deliberately not documenting the support, however as recent Ubuntu versions now ship with Ruby 3 (not 2.7) and Jekyll isn't showing any signs of moving towards supporting Ruby 3 we're not left with many alternatives.

This works around:

• https://github.com/github/pages-gem/issues/752
• https://github.com/envygeeks/pathutil/pull/5

If we're happy with this fix I'll also apply it to our other Jekyll projects (docs, style, competition-website).

[RealOrangeOne]

As part of this, it might be nice to add Ruby 3 to the CI matrix, so we make sure we keep support. I'm not sure what timeframe you imagine us switching the default install to 3 ( I too share the plague of trying to use older Ruby versions).

[PeterJCLaw]

As part of this, it might be nice to add Ruby 3 to the CI matrix, so we make sure we keep support.

Well that kinda implies it's supported ... which it isn't.

I'm not sure what timeframe you imagine us switching the default install to 3 ( I too share the plague of trying to use older Ruby versions).

At the moment, none. I don't think we can default to that until the production build (i.e: GitHub Pages itself) switches.

In theory we could move to the manual github pages (via Actions) mechanism, but even then I'm not keen on the production build involving the level of hacks needed here.

[PeterJCLaw]

Essentially I see this at the moment as being for those who know what they're doing and want to risk Ruby 3, rather than something we want to encourage people to use.

[PeterJCLaw]

Ignore the CircleCI failure -- we don't use CircleCI for this project any more.

[raccube]

We should switch over to building the site ourselves in GitHub Actions using the Publish to GitHub Pages action. This would allow us to use a more up to date version of Jekyll.