[KED-2034] Put multi-pipeline selector behind a flag

[richardwestenra]

Description

Put multi-pipeline selector behind a flag, to make it safe to release even while it's still a work-in-progress.

Now, even if you have multiple pipelines available, the selector UI will not be displayed unless you enable the flag.

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added new entries to the RELEASE.md file
• Added tests to cover my changes

Legal notice

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• I certify that the use of this contribution as authorised by the Apache 2.0 license does not violate the intellectual property rights of anyone else.

[bru5]

If you try enabling the flag, choosing a pipeline other than default, then disable the flag I think I'd expect it to reset back to the default pipeline. It doesn't seem to do that though, so there might be something missing here from the implementation?