Bump uk.org.webcompere:system-stubs-core from 2.0.3 to 2.1.8

Bumps [uk.org.webcompere:system-stubs-core](https://github.com/webcompere/system-stubs) from 2.0.3 to 2.1.8.
- [Release notes](https://github.com/webcompere/system-stubs/releases)
- [Changelog](https://github.com/webcompere/system-stubs/blob/main/History.md)
- [Commits](webcompere/system-stubs@system-stubs-parent-2.0.3...system-stubs-parent-2.1.8)

---
updated-dependencies:
- dependency-name: uk.org.webcompere:system-stubs-core
  dependency-version: 2.1.8
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

build.gradle

@@ -115,9 +115,9 @@ subprojects {
         testRuntimeOnly group: 'org.mockito', name: 'mockito-junit-jupiter', version: "${mockitoVersion}"
 
         if (JavaVersion.current() == JavaVersion.VERSION_1_8) {
-            testImplementation group: 'uk.org.webcompere', name: 'system-stubs-core', version: '2.0.3' // Last version supporting Java 8
+            testImplementation group: 'uk.org.webcompere', name: 'system-stubs-core', version: '2.1.8' // Last version supporting Java 8
         } else {
-            testImplementation group: 'uk.org.webcompere', name: 'system-stubs-core', version: '2.1.7'
+            testImplementation group: 'uk.org.webcompere', name: 'system-stubs-core', version: '2.1.8'
         }
 
         // Project Lombok dependency