Update kotest to v6

Author: renovate[bot]

gradle/libs.versions.toml

@@ -3,7 +3,7 @@ java = "21"
 kotlin = "2.2.21"
 coroutines = "1.10.2"
 flatlaf = "3.6.1"
-kotest = "5.9.1"
+kotest = "6.1.1"
 ignition = "8.1.20"
 jackson = "2.20.1"