Extract common jvm target value, add jvmTarget into documentation. Close #2157 (#2183)

README.md

@@ -82,6 +82,12 @@ detekt {
     filters = ".*/resources/.*,.*/build/.*"
     baseline = file("my-detekt-baseline.xml") // Just if you want to create a baseline file.
 }
+
+tasks {
+    withType<Detekt> {
+        this.jvmTarget = "1.8"
+    }
+}
 ```
 
 See [bintray](https://bintray.com/arturbosch/code-analysis/detekt) for releases and [artifactory](https://oss.jfrog.org/artifactory/webapp/#/artifacts/browse/tree/General/oss-snapshot-local/io/gitlab/arturbosch/detekt/detekt-cli/) for snapshots.

build.gradle.kts

@@ -103,10 +103,12 @@ subprojects {
         jacoco.toolVersion = jacocoVersion
     }
 
+    val projectJvmTarget = "1.8"
+
     tasks.withType<Detekt>().configureEach {
         exclude("resources/")
         exclude("build/")
-        jvmTarget = "1.8"
+        jvmTarget = projectJvmTarget
     }
 
     val userHome = System.getProperty("user.home")
@@ -164,7 +166,7 @@ subprojects {
     }
 
     tasks.withType<KotlinCompile>().configureEach {
-        kotlinOptions.jvmTarget = "1.8"
+        kotlinOptions.jvmTarget = projectJvmTarget
         // https://youtrack.jetbrains.com/issue/KT-24946
         kotlinOptions.freeCompilerArgs = listOf(
             "-progressive",