chore: remove pitest for mutation testing

In practice, I don't use it, and it blocks upgrading to Gradle 9:
#2041

Author: krzema12

github-workflows-kt/build.gradle.kts

@@ -11,7 +11,6 @@ plugins {
 
     // Code quality.
     id("io.gitlab.arturbosch.detekt")
-    id("info.solidsoft.pitest") version "1.15.0"
     id("org.jetbrains.kotlinx.binary-compatibility-validator") version "0.18.1"
 
     id("org.jetbrains.dokka") version "2.0.0"
@@ -78,10 +77,6 @@ tasks.formatKotlinMain {
     kotlinterConfig()
 }
 
-pitest {
-    junit5PluginVersion.set("1.1.0")
-}
-
 dokka {
     moduleName.set("github-workflows-kt")
 }