Bump system-stubs.version from 2.1.7 to 2.1.8

Bumps `system-stubs.version` from 2.1.7 to 2.1.8.

Updates `uk.org.webcompere:system-stubs-core` from 2.1.7 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-2.1.7...system-stubs-parent-2.1.8)

Updates `uk.org.webcompere:system-stubs-jupiter` from 2.1.7 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-2.1.7...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-patch
- dependency-name: uk.org.webcompere:system-stubs-jupiter
  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]

log4j-parent/pom.xml

@@ -138,7 +138,7 @@
     <pax-exam.version>4.14.0</pax-exam.version>
     <plexus-utils.version>3.6.0</plexus-utils.version>
     <spotbugs-annotations.version>4.9.3</spotbugs-annotations.version>
-    <system-stubs.version>2.1.7</system-stubs.version>
+    <system-stubs.version>2.1.8</system-stubs.version>
     <velocity.version>1.7</velocity.version>
     <xmlunit.version>2.10.0</xmlunit.version>