Markdown reader: don't allow ^ at beginning of link or image label.

This is reserved for footnotes.
Fixes a regression introduced by 0a93acf.

Closes #7723.

src/Text/Pandoc/Readers/Markdown.hs

@@ -1775,8 +1775,7 @@ endline = try $ do
 -- a reference label for a link
 reference :: PandocMonad m => MarkdownParser m (F Inlines, Text)
 reference = do
-  -- guardDisabled Ext_footnotes <|> notFollowedBy' (string "[^")
-  -- guardDisabled Ext_citations <|> notFollowedBy' (string "[@")
+  guardDisabled Ext_footnotes <|> notFollowedBy' (string "[^")
   withRaw $ trimInlinesF <$> inlinesInBalancedBrackets
 
 parenthesizedChars :: PandocMonad m => MarkdownParser m Text

test/command/7723.md

@@ -0,0 +1,8 @@
+```
+% pandoc -t native
+Bug![^1]
+
+[^1]: Note.
+^D
+[ Para [ Str "Bug!" , Note [ Para [ Str "Note." ] ] ] ]
+```