Make the 2024 edition the default once 1.85 hits stable

[fmease]

Rust 2024 is stable since 1.83 (which can be selected on the playground). will be stable in 1.85.
Selecting it should no longer switch to nightly.

The patch should look very similar to PR #739.
I wanted to submit a PR myself but I failed to properly build the playground locally
outside of a Docker container and gave up prematurely because I didn't have the nerve
and time to set up a Docker container or to look into the issues I encountered. Sorry.

[shepmaster]

Don’t worry, I’ll get to it.

Building the playground frontend and backend do not require Docker, for what it’s worth, only actually running a command.

I think you’ve jumped the gun by a month or two though?

As planned, we recently merged the feature-complete Rust 2024 edition to the release train for Rust 1.85.

— https://blog.rust-lang.org/2024/11/27/Rust-2024-public-testing.html

[fmease]

I think you’ve jumped the gun by a month or two though?

Oops, you are absolutely right, how embarrassing! ^^ This shows that I'm through and through nightly-brained (Rust 2024 is only "stable" on nightly / 1.85-nightly).