Bump uk.org.webcompere:system-stubs-junit4 in /modules/grafana

Bumps [uk.org.webcompere:system-stubs-junit4](https://github.com/webcompere/system-stubs) from 2.1.6 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-2.1.6...system-stubs-parent-2.1.8)

---
updated-dependencies:
- dependency-name: uk.org.webcompere:system-stubs-junit4
  dependency-version: 2.1.8
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

modules/grafana/build.gradle

@@ -6,7 +6,7 @@ dependencies {
     testImplementation 'org.assertj:assertj-core:3.27.3'
     testImplementation 'io.rest-assured:rest-assured:5.5.5'
     testImplementation 'io.micrometer:micrometer-registry-otlp:1.15.1'
-    testImplementation 'uk.org.webcompere:system-stubs-junit4:2.1.6'
+    testImplementation 'uk.org.webcompere:system-stubs-junit4:2.1.8'
 
     testImplementation platform('io.opentelemetry:opentelemetry-bom:1.51.0')
     testImplementation 'io.opentelemetry:opentelemetry-api'