singlehtml: deprecate the 'fix_refuris' helper function (#13037)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

doc/extdev/deprecated.rst

@@ -22,6 +22,11 @@ The following is a list of deprecated interfaces.
      - Removed
      - Alternatives
 
+   * - ``sphinx.builders.singlehtml.SingleFileHTMLBuilder.fix_refuris``
+     - 8.2
+     - 10.0
+     - N/A
+
    * - ``sphinx.util.FilenameUniqDict``
      - 8.1
      - 10.0