Update gem files.

[rvandermeulen]

This should clean up the Dependabot alerts we've currently got.

[rvandermeulen]

Note that longer-term, we should probably migrate away from using github-pages as upstream has no intent of supporting jekyll 4 (see github/pages-gem#651) and it's not clear to me how supported 3.x is at this point - though I'll guess the answer is "it isn't".

[parkr]

it's not clear to me how supported 3.x is at this point - though I'll guess the answer is "it isn't".

Hi! I am a maintainer of Jekyll. We're not adding new features to Jekyll 3, but I am still actively maintaining it for security patches. If you find something, feel free to send it my way on jekyll/jekyll.

I definitely recommend using Actions to build – you can still deploy to Pages and use their serving infrastructure even if you have a custom build. :)