Revert oklch usage in core

[uhafner]

#10078 introduces oklch usage in Jenkins core. This PR caused a major regression, see JENKINS-75242 that affects all plugins that use trend charts that are based on the echarts plugin. Some examples are the popular junit, warnings, and coverage plugins.

Since nobody corresponded on the JENKINS-75242 yet, it makes sense to revert the original change until a solution has been found in core or the affected plugins.

Testing done

Manual testing of the charts in my instance. Now all colors are working again when hovering over the chart.

Proposed changelog entries

N/A

Proposed changelog category

/label skip-changelog

Proposed upgrade guidelines

N/A

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Desired reviewers

@timja @janfaracik

Before the changes are marked as ready-for-merge:

Maintainer checklist

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[timja]

Looks solvable in echarts-api: apache/echarts#20757

If someone has time to give it a go, otherwise we can revert till sorted.

[janfaracik]

Hey, apologies for not getting back.

I'll take a look and try to fix it in the plugin.

Thanks

[janfaracik]

https://github.com/jenkinsci/echarts-api-plugin/pull/380/files

Opened in draft, needs some more testing