use --release flag of recent java versions

[soloturn]

since java9 java supports a release flag for building to make sure the code is not using newer APIs. this is supported in maven and gradle:

• https://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-release.html
• https://docs.gradle.org/current/userguide/building_java_projects.html

using this flag revealed two challenges:

• javac -target -source flags are not good enough guarantee compatiblity with old versions. use --release instead.
• plantuml uses java.time, introduced in java8. it was built always for java7 and the tools did not notice that installing and using real java7 gives compile and runtime errors.

would be cool if this does not happen any more in future.

[soloturn]

compiling with java17, gradle, setting release to java7 fails immediate, the source code currently needs java8:

> Task :compileJava
warning: [options] source value 7 is obsolete and will be removed in a future release
warning: [options] target value 7 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
/srv/http/repos/plantuml/src/net/sourceforge/plantuml/project/time/Month.java:38: error: package java.time.format does not exist
import java.time.format.TextStyle;
                       ^
/srv/http/repos/plantuml/src/net/sourceforge/plantuml/project/time/DayOfWeek.java:39: error: package java.time.format does not exist
import java.time.format.TextStyle;
                       ^
/srv/http/repos/plantuml/src/net/sourceforge/plantuml/project/time/Month.java:61: error: package java.time does not exist
        private java.time.Month getJavaTimeMonth() {
                         ^

[soloturn]

i installed java7, and it really fails. now i see what the --release flag is for. it effectively gives the same errors without bothering to really install the old java version. a cool flag, i like it.