[JENKINS-73330] Fix dropdowns and tooltips on HDR displays

[janfaracik]

See JENKINS-73330.

Safari on macOS behaves a little differently to every other browser in that it turns on HDR when using the brightness(...) filter in CSS. This causes dropdowns, tooltips and dialogs to appear over exposed (and literally brighter) when working on a HDR display. It's pretty painful.

This PR fixes that, by removing the brightness() filter, and instead just setting the background on those elements to a semi translucent colour.

See #9649 for a fix to dialogs.

Testing done

• Dropdowns and tooltips appear as expected, similar to how they did before, just not overblown

Proposed changelog entries

• Fix dropdown and tooltip brightness on HDR displays.

Proposed upgrade guidelines

N/A

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Desired reviewers

@jenkinsci/sig-ux

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[janfaracik]

/label web-ui

[NotMyFault]

Good catch!

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!