Fix Java 8 compatibility issue

[amottier]

• Update the pom.xml to build a version compatible with Java 8 using AdoptopenJDK 11.
• maven.compiler.release (javac option --release) is used instead of maven.compiler.source and maven.compiler.target (javac options -source and -target) to produce class files link against the appropriate platform version (see http://openjdk.java.net/jeps/247).

[cesarsotovalero]

Hi @amottier.
I'm looking at this SO answer.
We want to be compatible with Java 8, but build with Java 11+ (taking care of API incompatibility changes at the same time).

It seems that the solution using a profile in the SO answer is a good approach.

[amottier]

@cesarsotovalero as far as I understand the solution described in SO answer cover the need of being able to build both with JDK 8 and 9+.

If the need for depclean is to be compatible:

• for execution with JDK 8 and higher
• for compilation with JDK 9+ only (not compatible with JDK 8 or lower)

I think that the solution I suggest is sufficient.

Actually I tested my PR by building it with OpenJDK 11 and then using the depclean plugin with both OpenJDK 8 and 11 and it run flawlessly. So I'm quite confident that my solution fix the issue.

[cesarsotovalero]

@amottier I also tested the changes. It builds successfully with JDKs 8, 11, and 15.
I'm merging your PR.
Thanks for your contribution 👍