Add jdk 21

[sleski]

Description:
19.09.2023 new LTS was released - jdk 21, please add possibility to use it via github-actions.

Justification:
new Java LTS was released.

Are you willing to submit a PR?

[sleski]

My mistake, we do not need this.

[hertg]

For anybody stumbling here. The JDK 21 has not yet been released for the temurin distribution. For the time being, the early access build can be used by specifying 21-ea.

- uses: actions/setup-java@v3
  with:
    distribution: temurin
    java-version: 21-ea

https://adoptium.net/blog/2023/08/early-access-builds/

[beatngu13]

Regarding Temurin 21:

https://adoptium.net/en-GB/blog/2023/09/temurin21-delay/

[garydgregory]

Temurin 21 looks to be available on most platforms: adoptium/temurin#8