Compile Java 11 artifacts with --release

[gregjotau]

Closes #2032.

Replace the separate Java source/target settings with maven.compiler.release=11. This preserves the Java 11 bytecode and language baseline while also preventing accidental linkage to JDK APIs introduced after Java 11.

Verification:

• mvn clean test -DskipITs
• compiler reports [debug release 11]
• the system modules ... -source 11 warning is gone
• 617 tests pass, with 4 skipped

[gemini-code-assist]

Code Review

This pull request updates the Maven compiler configuration in pom.xml to use the <maven.compiler.release> property instead of the separate <maven.compiler.source> and <maven.compiler.target> properties. I have no further feedback to provide as there are no review comments.

[jeandersonbc]

Hey @gregjotau, thank you not only for raising the issue but providing this PR (along with the Maven reference). We appreciate your time checking it! I also just verified myself for the warning (mvn clean compile) and using the release option. Changes look good to me!

[gregjotau]

Great, we integrate Adyen into reai.no (accounting system) now and we want to contribute and improve all third party libraries we use.

Especially important for us that dependencies are modularised in such a way that we bring in as little surface as possible :)

thanks @jeandersonbc !