Upgrade to LTS JDK21

[n2o]

• Change version to JDK 21 in Dockerfile and Gradle

[NyorJa]

hello @n2o please allow me to take this issue

[n2o]

You're welcome :-) But please use JDK 21, since 21 is the latest LTS JDK. Thanks!

[NyorJa]

yes, will use jdk21

[NyorJa]

hi @n2o, im encountering an issue in unit test when using jdk21 in archtests package. I will submit my PR so that you can check