Build: Move java related source/target compatibility into java block

This solves the 'Assignment is not used' build warnings on the
'fluxc-annotations' and 'fluxc-processor' Java related modules.

Author: ParaskP7

fluxc-annotations/build.gradle

@@ -4,13 +4,13 @@ plugins {
 }
 
 java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+
     withSourcesJar()
     withJavadocJar()
 }
 
-sourceCompatibility = JavaVersion.VERSION_1_8
-targetCompatibility = JavaVersion.VERSION_1_8
-
 project.afterEvaluate {
     publishing {
         publications {

fluxc-processor/build.gradle

@@ -6,13 +6,13 @@ plugins {
 }
 
 java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+
     withSourcesJar()
     withJavadocJar()
 }
 
-sourceCompatibility = JavaVersion.VERSION_1_8
-targetCompatibility = JavaVersion.VERSION_1_8
-
 dependencies {
     implementation fluxcAnnotationsProjectDependency
     implementation sharedLibs.google.autoService