Raise the minimum Java version to 11.

[Moderocky]

Description

Raises Skript's compile target from Java 8 to Java 11. Raises the test runners (and test server tasks) to use Java 11.

With 2.9 we are dropping support for Java 8. We will still support Minecraft versions 1.13.2+ with this Skript version, as long as they are running Java 11.

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Target Minecraft Versions: any Requirements: Java 11 Related Issues: #4625 & closes #1321

[Moderocky]

This is probably to be the last thing merged before 2.9, in case anybody needs to make an unofficial Java 8 build by checking out the final commit.

Ironically, the first post-2.9 feature commit will probably be bumping the version again to Java 17.

[TheLimeGlass]

Make it Java 17. Java 11 doesn't benefit much. Other than less MC versions from 1.17