Update top-level "bugs" menu item

It was still pointing to the Bugzilla that we are no longer using. Point to the Github issue query instead.

Author: ftynse

website/config.toml

@@ -94,7 +94,7 @@ disablePathToLower = true
 [[menu.main]]
     name = "Bugs"
     weight = 20
-    url = "https://bugs.llvm.org/buglist.cgi?bug_status=__open__&list_id=177877&order=changeddate%20DESC%2Cpriority%2Cbug_severity&product=MLIR&query_format=specific"
+    url = "https://github.com/llvm/llvm-project/issues?q=is%3Aissue%20state%3Aopen%20label%3Amlir"
 
 [[menu.main]]
     name = "Logo Assets"