javac --release to build for java8, with java11+

javac since java9 has a new flat --release to build a binary which can be run
with an older java. the public API of that older release must be followed as
well.

the flags used before, -source, -target do not do this. therefor it got
unnoticed that plantuml did not compile and run in java7 any more, despite
beeing compiled for for java7.

https://github.com/plantuml/plantuml/issues/898

build.gradle.kts

@@ -18,8 +18,6 @@ dependencies {
 
 group = "net.sourceforge.plantuml"
 description = "PlantUML"
-java.sourceCompatibility = JavaVersion.VERSION_1_8
-java.targetCompatibility = JavaVersion.VERSION_1_8
 
 java {
   withSourcesJar()
@@ -51,6 +49,10 @@ sourceSets {
   }
 }
 
+tasks.compileJava {
+	options.release.set(8)
+}
+
 tasks.withType<Jar> {
   manifest {
     attributes["Main-Class"] = "net.sourceforge.plantuml.Run"

pom.xml

@@ -56,8 +56,7 @@
   </parent>
   <properties>
     <finalName>${project.artifactId}-${project.version}</finalName>
-    <maven.compiler.source>1.8</maven.compiler.source>
-    <maven.compiler.target>1.8</maven.compiler.target>
+    <maven.compiler.release>8</maven.compiler.release>
     <main.class>net.sourceforge.plantuml.Run</main.class>
   </properties>
   <dependencyManagement>