Fix mixin custom job_prefix usage with job selector

[Polygens]

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What this PR does

Usage of job_prefix was not used consistently in the dashboard-utils, jobSelector results in an invalid selector for custom installs of Mimir that do not prefix the job with ($namespace)/.

This PR aligns jobSelector with jobMatcher. Output of the mixins compiled remains identical.

Which issue(s) this PR fixes or relates to

No issue exists

Checklist

• [ ] Tests updated.
• [ ] Documentation added.
• [X] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [ ] about-versioning.md updated with experimental features.

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