Compile with Java 21

[dilyanpalauzov]

With Java 21 the current code could not be compiled, so I made random chanegs, without understanding these, until finally the software build.

According to https://docs.gradle.org/current/userguide/compatibility.html: Java 21 requires Gradle 8.5

[fwolter]

Did you try to upgrade to gradle 9, or is there too much to change?

[dilyanpalauzov]

I guess you want to ask why I have not upgraded to Gradle 9. I tried to update different things at the same time - Gradle version, biz.aQute.bnd.builder version, build.gradle file. Upgrading to Gradle 9, without doing the other changes, has not worked. I ended in a combination, which works.

Once the herein included components - bnd and gradle - are updated, e.g. to version 6.4.0 / 8.11.1, later other version jumps can be done, resulting possibly smaller changes.

But looking at the Gradle/Java compatibility matrix now, I realize that before the current change up to Java 13 could be used.

With the current change Java versions up to 22 can be used. Switching to Gradle 9 would mean that Java 16 and lower versions will not work, but Java 25 will work.

[fwolter]

Seems reasonable. Don't know why the CI build does not start...

[dilyanpalauzov]

Similar to the idea of the current change #238 proposes a switch to Gradle 8.3.