Bump spock-core from 2.1-groovy-3.0 to 2.3-groovy-4.0

Bumps spock-core from 2.1-groovy-3.0 to 2.3-groovy-4.0.

---
updated-dependencies:
- dependency-name: org.spockframework:spock-core
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

plugin/build.gradle.kts

@@ -38,7 +38,7 @@ dependencies {
   api("de.gesellix:docker-client:2025-05-17T19-00-00")
 
   testImplementation(localGroovy())
-  testImplementation("org.spockframework:spock-core:2.3-groovy-3.0")
+  testImplementation("org.spockframework:spock-core:2.3-groovy-4.0")
   testImplementation("cglib:cglib-nodep:3.3.0")
 
   // see https://docs.gradle.org/current/userguide/test_kit.html