forge-topics-setup-buffer: Avoid new warning from Emacs 31

Using a initarg instead of the respective slot name with `oref'
et al. now results in a warning.  We want to stick to using keywords
in `forge-topics-setup-buffer's PARAMS argument, so we have to mangle
the symbols.

Author: tarsius

lisp/forge-topics.el

@@ -130,7 +130,7 @@ Must be set before `forge-topics' is loaded.")
                            (buffer-local-value 'forge--buffer-topics-spec buf)))
                      ((clone forge-list-buffer-default-topic-filters)))))
     (while-let ((key (pop params)))
-      (eieio-oset spec key (pop params)))
+      (eieio-oset spec (intern (substring (symbol-name key) 1)) (pop params)))
     (unless (oref spec type)
       (oset spec type 'topic))
     (forge--cast-topics-spec-state spec)