Fix build warnings

Fix the following build warnings:

~~~
> Task :compileKotlin
w: Flag is not supported by this version of the compiler: -Xuse-experimental=kotlin.Experimental
~~~

~~~
> Task :virtual-device:compileKotlin
w: Argument -Xopt-in is deprecated. Please use -opt-in instead
~~~

Signed-off-by: Diego Rondini <diego.rondini@kynetics.it>

build.gradle

@@ -140,7 +140,7 @@ project(':virtual-device'){
     mainClassName = 'org.eclipse.hara.ddiclient.virtualdevice.MainKt'
 
     tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
-        kotlinOptions.freeCompilerArgs += ["-Xopt-in=kotlin.RequiresOptIn"]
+        kotlinOptions.freeCompilerArgs += ["-opt-in=kotlin.RequiresOptIn"]
     }
 }
 
@@ -253,10 +253,6 @@ test.finalizedBy stopHawkbitServer
 group 'org.eclipse.hara.hara-ddiclient'
 version app_version
 
-tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
-    kotlinOptions.freeCompilerArgs += ["-Xuse-experimental=kotlin.Experimental"]
-}
-
 test {
     useTestNG()