Fix missing margin face on display prop in erc-stamp

* lisp/erc/erc-stamp.el (erc-insert-timestamp-right): Explicitly add
face to right-hand stamp's `display' margin string.  Commit d24b10ca
"Introduce 'margin' face for window margin background" did away with the
"fall-through" behavior in which a margin spec's string that lacks its
own face property inherited whatever the associated `display' char
happened to have.  (Bug#80693)

;; The old language in "(elisp) Display Margins" said "if the
;; string to be displayed in the margin doesn't specify a face, its
;; face is determined using the same rules and priorities as it is
;; for strings displayed in the text area (see Displaying Faces),"
;; and that "if this results in undesirable 'leaking' of faces into
;; the margin, make sure the string has an explicit face specified
;; for it."  It seems ERC and likely more than a few other packages
;; never came to heed this guidance and have erroneously depended
;; on such behavior for years if not decades.
;;
;; * test/lisp/erc/resources/fill/snapshots/merge-01-start.eld:
;; * test/lisp/erc/resources/fill/snapshots/merge-02-right.eld:
;; * test/lisp/erc/resources/fill/snapshots/merge-wrap-01.eld:
;; * test/lisp/erc/resources/fill/snapshots/merge-wrap-indicator-pre-01.eld:
;; * test/lisp/erc/resources/fill/snapshots/monospace-01-start.eld:
;; * test/lisp/erc/resources/fill/snapshots/monospace-02-right.eld:
;; * test/lisp/erc/resources/fill/snapshots/monospace-03-left.eld:
;; * test/lisp/erc/resources/fill/snapshots/monospace-04-reset.eld:
;; * test/lisp/erc/resources/fill/snapshots/spacing-01-mono.eld: Update
;; snapshot.

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