[Gradle & Actions] Use Java 11 for compiling and checks instead of Java 8

[coderbot16]

We can use the compilerArgs option to still make sure that our code is compatible with Java 8 even though we're compiling on Java 11:

compileJava {
    options.compilerArgs.addAll(['--release', '8')]
}

Unfortunately, that fails on Java 8 because the flag was only added in Java 9. However, this blog post has a good solution:

// If on JDK 9+, verify project cross-compiles on its 'sourceCompatible' JVM version (see https://github.com/melix/mrjar-gradle/blob/master/jdks.gradle)
if (project.hasProperty('crossCompile')) {
    if (JavaVersion.current().java9Compatible) {
        project.afterEvaluate {
            tasks.withType(JavaCompile) {
                def version = compat(sourceCompatibility)
                project.logger.info("Configuring $name to use --release $version")
                options.compilerArgs.addAll(['--release', version])
            }
        }
    } else {
        project.logger.warn("-PcrossCompile not supported prior to JDK 9; using JDK ${JavaVersion.current()}")
    }
}

// This function converts 1.8 -> 8
static String compat(String src) {
    if (src.contains('.')) {
        src.substring(src.lastIndexOf('.')+1)
    } else {
        src
    }
}

While that code supports cross compiling to any version, in reality we only care about cross compiling to Java 8 and/or compiling to Java 8 on Java 8 at the moment so we can just hardcode that part in and remove the warning when cross compiling isn't supported.

[ramidzkh]

FabricMC/fabric@c2f49c1 is much smaller