Deprecate the non-permanent CLI build flags

[bep]

Hey clutter up the CLI help doc and doesn't really belong there. The flags below feels more like permanent site configuration, and if you really need to set the value on the command line, you can do it via an OS environment variable, e.g:

HUGO_UGLYURLS=true hugo

The CLI flags to deprecate and then to remove in a future Hugo version are:

• canonifyURLs
• pluralizeListTitles
• preserveTaxonomyNames
• uglyURLs

[github-actions]

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