[JENKINS-73539] Disable YUI by default

[timja]

See JENKINS-73539.

This is the culmination of a lot of work over the years to slowly replace existing components that were built with YUI to alternatives.
A few examples:

• Replace YUI tooltips with Tippy.js #6408
• Use standard dropdowns for autocomplete #9453
• Replace usages of YUI framework by simple JS/CSS #9395
• [JENKINS-67396] Update link and breadcrumb dropdowns #6084
• [JENKINS-64291] Fix drag & drop issues replacing the drag & drop library #5177

A number of plugins and core changes have been prepared to make this possible.
See the tracking spreadsheet: https://docs.google.com/spreadsheets/d/1UjvtFmNmEdjMN5DUoFxJfBryA8q-E5_HwOzVKbVG9b0/edit?gid=969462704#gid=969462704

We aren't removing YUI at this time as @MarkEWaite requested that we wait till after the next LTS cutoff so-as vendors can change the default back if they still require YUI in their commercial products.

Testing done

Checked elements in the browser tools, by default no yui is there.

Disabled the flag in user settings and yui appears

Ran System.setProperty("jenkins.model.experimentalflags.RemoveYuiUserExperimentalFlag.defaultValue", "false") in the script console and yui appears without having to set a flag.

Clicked around and no issues found.
ATH is successful: jenkinsci/acceptance-test-harness#1861

PCT is successful: jenkinsci/bom#4084.

Proposed changelog entries

• Disable the Yahoo! User Interface library by default

Proposed upgrade guidelines

N/A

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2. The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
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Desired reviewers

@mention

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

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[mawinter69]

Not sure if this is the right approach. The flag is a user specific thing, so when a plugin is installed that still requires YUI, it means that each and every user of that instance needs to enable it in their personal settings.
I would remove that experimental flag and instead introduce a system property that allows to define if YUI should be enabled or not. That way the admin of an instance can define at startup that YUI is still needed.

[janfaracik]

LGTM <3 Been a long time coming for this one.

[timja]

/label ready-for-merge

---

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

Thanks!

[mawinter69]

good enough
Does it need the upgrade-guide-needed label?

[timja]

good enough Does it need the upgrade-guide-needed label?

lets add just in case