Adjust PIT gradle plugin configuration (#2903)

Starting with PIT 1.6.7 it is just enough to have kotest-extensions-pitest on (test) classpath

Over more that parameter is already deprecated and "marked for deletion". See:
hcoles/pitest#900
szpak/gradle-pitest-plugin#277

documentation/docs/extensions/pitest.md

@@ -22,7 +22,7 @@ After doing that, we need to inform Pitest that we're going to use `Kotest` as a
 ```kotlin
 // Assuming that you have already configured the Gradle/Maven extension
 configure<PitestPluginExtension> {
-    testPlugin.set("Kotest")    // <-- Telling Pitest that we're using Kotest
+    // testPlugin.set("Kotest")    // needed only with old PIT <1.6.7, otherwise having kotest-extensions-pitest on classpath is enough
     targetClasses.set(listOf("my.company.package.*"))
 }
 ```