chore: Update to Ruby v3.3 and jekyll v4.3

[matthewfeickert]

This currently has some warnings and issues with "Card image cap" on some people in the picture grids, but this is a start to get things going. Early feedback is welcome!

This PR supersedes part of PR #1728, but doesn't address the docs in the same way and so PR #1728 should remain open to get rebased after this.

Squash and merge commit message

* Update lower bounds on jekyll and jekyll-feed.
* Update pixi env to use Ruby v3.3 and rebuild lock file.
* Rebuild Gemfile.lock for Ruby v3.3.
* Update CI workflow to use Ruby v3.3.
* Update dev docs to use Ruby v3.3.

[matthewfeickert]

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