Administration: Remove multiple viewport meta tags from mobile pages.

In addition, add the `wp_admin_viewport_meta()` function, paired to the `admin_viewport_meta` filter to control attributes of the meta tag.

Fixes #47369.
Props BettyJJ, mukesh27, SergeyBiryukov, ajayghaghretiya1, msaggiorato, talldanwp, davidbaumwald, donmhico, audrasjb.


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