Update kotest to v6

Author: renovate[bot]

gradle/libs.versions.toml

@@ -16,7 +16,7 @@ binaryCompatibility = "0.18.1"
 
 # --- Testing ---
 junit = "5.14.0"             # https://junit.org/junit5
-kotest = "5.9.1"             # https://kotest.io
+kotest = "6.0.4"             # https://kotest.io
 kotlintest = "3.4.2"         # https://mvnrepository.com/artifact/io.kotlintest/kotlintest-core
 assertj = "3.27.6"           # https://assertj.github.io/doc
 mockito = "5.20.0"           # https://mvnrepository.com/artifact/org.mockito/mockito-core