Unable to install Paper 1.21

[JoshLoecker]

Description

When selecting Paper as the server type and 1.21 as the version, I get the following error notification

Setting up Minecraft server Survival
Instance creation failed: An error occurred (Internal Error): Could not find a paper server jar for version 1.21

[seliayeu]

Looks like the issue is that we don't support experimental branches (the code in get_paper_jar_url filters by default channel instead of experimental).
Whether or not we decide to support it, having unsupported versions appear as valid on the frontend is probably not a good idea, though.

[JoshLoecker]

The option to install experimental branches would be greatly appreciated, even if it meant enabling an "experimental" flag, UI button, etc. to allow them :)