Upgrade to Java 21

[frcroth]

URL of deployed dev instance (used for testing):

• https://java.webknossos.xyz

Steps to test:

• @fm3 you can test installing java as in the DEV_INSTALL

Issues:

• fixes Use newer JVMs in images + installation guides #7491

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(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Adapted wk-libs python client if relevant API parts change
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases
• Needs datastore update after deployment

[fm3]

Works for me :) @normanrz could you also have a look? Does this make sense for mac?
Do you think there may be disadvantages of using the eclipse temurin package and not openjdk directly for local development?

[normanrz]

Please also update https://github.com/scalableminds/dockerfiles/blob/master/sbt/Dockerfile