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 :aztec:compileDebugKotlin FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':aztec:compileDebugKotlin'.
> Inconsistent JVM-target compatibility detected for tasks 'compileDebugJavaWithJavac' (1.8) and 'compileDebugKotlin' (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

@@ -29,6 +29,7 @@ allprojects {
     }
     tasks.withType(KotlinCompile).all {
         kotlinOptions {
+            jvmTarget = JavaVersion.VERSION_1_8
             allWarningsAsErrors = true
         }
     }