Deprecating Functions.singletonList

[jglick]

See #6732 (comment).

Proposed changelog entries

• Deprecating Functions.singletonList which is not normally needed in JEXL.

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[jglick]

Not sure why it would matter. It could be considered retroactively deprecated since the beginning of the Hudson project. Anyway the call sites are in Jelly sources where no IDE will be able to display this information.

[timja]

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

Thanks!

[MarkEWaite]

I believe that the emailext-template-plugin groovy UI script that calls h.singletonList(it) needs to be changed in order to fix JENKINS-69153. jenkinsci/emailext-template-plugin#71 has been submitted to the plugin with a proposed fix.