Build: Add jvm target to point to java 8 to kotlin options

This change is necessary because otherwise the build fails with the
below exception:

------------------------------------------------------------------------

> Task :fluxc-processor:kaptGenerateStubsKotlin FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':fluxc-processor:kaptGenerateStubsKotlin'.
> Inconsistent JVM-target compatibility detected for tasks 'compileJava'
 (1.7) and 'kaptGenerateStubsKotlin' (17).

  Consider using JVM Toolchain: https://kotl.in/gradle/jvm/toolchain
  Learn more about JVM-target validation: https://kotl.in/gradle/jvm/
   target-validation

Author: ParaskP7

build.gradle

@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
 plugins {
     // Those declarations are just a workaround for a false-positive Kotlin Gradle Plugin warning
     // https://youtrack.jetbrains.com/issue/KT-46200
@@ -57,6 +59,12 @@ allprojects {
         parallel = true
         debug = false
     }
+
+    tasks.withType(KotlinCompile).all {
+        kotlinOptions {
+            jvmTarget = JavaVersion.VERSION_1_8
+        }
+    }
 }
 
 ext {

fluxc-annotations/build.gradle

@@ -8,8 +8,8 @@ java {
     withJavadocJar()
 }
 
-sourceCompatibility = JavaVersion.VERSION_1_7
-targetCompatibility = JavaVersion.VERSION_1_7
+sourceCompatibility = JavaVersion.VERSION_1_8
+targetCompatibility = JavaVersion.VERSION_1_8
 
 project.afterEvaluate {
     publishing {

fluxc-processor/build.gradle

@@ -10,8 +10,8 @@ java {
     withJavadocJar()
 }
 
-sourceCompatibility = JavaVersion.VERSION_1_7
-targetCompatibility = JavaVersion.VERSION_1_7
+sourceCompatibility = JavaVersion.VERSION_1_8
+targetCompatibility = JavaVersion.VERSION_1_8
 
 dependencies {
     implementation fluxcAnnotationsProjectDependency