Update the minimum version of Jenkins used by RealJenkinsRule to prevent detached plugins being dragged in.

[jtnord]

As observed in https://github.com/jenkinsci/jenkins/pull/9351/checks?check_run_id=25813003911 the RealJenkinsRule was targetting an version of Jenkins core that required detached plugins.

This updates the version in the RealJenkinsRuleInit plugin to a version that does not require detached plugins.

Whilst this is an interim solution to the problem, the currently selected version made no sense to me as it was lower than the Jenkins version specified for the actual test harness.

Longer term (if needed) the test HPI could be unpacked, and have its MANIFEST.mf optionally updated to match the Jenkins version under test

incremental consumed in https://github.com/jenkinsci/jenkins/pull/9351

see also #780

Testing done

Submitter checklist

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