fix(workflows): use Java 21 to build

[tgbhy]

This just set the runtime_version of shared-ci.yml to Java 21.

[zml2008]

Why?

[tgbhy]

On #1253 the Adventure/5.0.0 use Java 21.
So, when building, the workflows currently use Java 17 to build which cause an error for incorrect JVM setup, so this PR aims to fix that so you can see the real errors to fix to have a functionnal build.
And when everything will be fixed, the CI will be able to work correctly.

I did tests on my GitHub before sending this PR, I just send unnecessary commits so I re-created this fork when everything was good for me.

[kezz]

Thanks for the PR but I'm going to close this as Adventure 5.x is still WIP at this point - it doesn't compile anyway at this time.