[BUG] polaris-core has hard compile-time dependencies on JDK 21

[adutra]

Describe the bug

While Polaris build requires Java 21+, the polaris-core module is built with source and target level = 11.

I suppose that this has been decided in order to make it possible to include polaris-core in applications running with Java 11.

However, the code in main sources makes frequent usage of JDK methods that were introduced after Java 11, such as:

• Stream.toList() (Java 16)
• List.getFirst() and List.getLast() (Java 21)

This makes the compiled artifact effectively unusable, if included in an application running with Java 11.

Is this a possible security vulnerability?

• yes -- if yes, stop here and contact security@polaris.io instead
• no

Expected behavior

The polaris-core module should not make use of any JDK API that was introduced after Java 11. Ideally, there should be a build task verifying this.

Alternatively, we should consider lifting the restriction of level 11 in polaris-core.

[adutra]

There is also List.reversed() (Java 21).

[eric-maynard]

@snazy Did we decide to just raise the version to 21? Or how do we want to handle this?

[snazy]

We haven't decided anything yet. Naively, I'd suggest 21 and opt for 11 for code that can be pulled in by clients.
I'll open a discussion.

[snazy]

#100