export back-ends: Fix leftover downcase :custom-id properties

* lisp/ox-html.el (org-html-format-headline--wrap, org-html-headline):
  Use :CUSTOM_ID, not :custom-id.
* contrib/lisp/ox-deck.el (org-deck-toc): Use :CUSTOM_ID,
  not :custom-id.

contrib/lisp/ox-deck.el

@@ -273,7 +273,7 @@ Note that the wrapper div must include the class \"slide\"."
 	   (if (and class (string-match-p "\\<slide\\>" class))
 	       (format
 		"<a href='#outline-container-%s'>%s</a>"
-		(or (org-element-property :custom-id headline)
+		(or (org-element-property :CUSTOM_ID headline)
 		    (mapconcat
 		     'number-to-string
 		     (org-export-get-headline-number headline info) "-"))

lisp/ox-html.el

@@ -1771,7 +1771,7 @@ holding contextual information."
 	 (text (org-export-data (org-element-property :title headline) info))
 	 (tags (and (plist-get info :with-tags)
 		    (org-export-get-tags headline info)))
-	 (headline-label (or (org-element-property :custom-id headline)
+	 (headline-label (or (org-element-property :CUSTOM_ID headline)
 			     (concat "sec-" (mapconcat 'number-to-string
 						       headline-number "-"))))
 	 (format-function (cond
@@ -1843,7 +1843,7 @@ holding contextual information."
 	     (level1 (+ level (1- org-html-toplevel-hlevel))))
 	(format "<div id=\"%s\" class=\"%s\">%s%s</div>\n"
 		(format "outline-container-%s"
-			(or (org-element-property :custom-id headline)
+			(or (org-element-property :CUSTOM_ID headline)
 			    section-number))
 		(concat (format "outline-%d" level1) (and extra-class " ")
 			extra-class)