fix: show swapfile warning as a warning (#28972)

The new default SwapExists autocommand displays warning text (W325) but
does not use the WarningMsg highlight group as other warnings do. Use
the WARN log level when displaying this warning.

(cherry picked from commit e71713ba2b)

runtime/lua/vim/_defaults.lua

@@ -266,7 +266,10 @@ do
         return
       end
       vim.v.swapchoice = 'e' -- Choose "(E)dit".
-      vim.notify(('W325: Ignoring swapfile from Nvim process %d'):format(info.pid))
+      vim.notify(
+        ('W325: Ignoring swapfile from Nvim process %d'):format(info.pid),
+        vim.log.levels.WARN
+      )
     end,
   })