Better guide users when deleting other users in single- and multisites; copy 10502

[joedolson]

This is a copy of #10502 with changes applied from @afercia and my own reviews, because I was unable to merge to that PR.

It does not include the switch of strings to use _n(), as I'm not convinced that the churn in strings is helpful.

Trac ticket: https://core.trac.wordpress.org/ticket/56914

Use of AI Tools

None

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