Update Coq flags in _CoqProject.

_CoqProject

 -Q theories lrust
-# We sometimes want to locally override notation and there is no good way to do that with scopes.
+# We sometimes want to locally override notation (e.g. in proofmode/base.v, bi/embedding.v), and there
+# is no good way to do that with scopes.
 -arg -w -arg -notation-overridden
 # non-canonical projections (https://github.com/coq/coq/pull/10076) do not exist yet in 8.9.
 -arg -w -arg -redundant-canonical-projection
 # change_no_check does not exist yet in 8.9.
 -arg -w -arg -convert_concl_no_check
+# "Declare Scope" does not exist yet in 8.9.
+-arg -w -arg -undeclared-scope
+# We have ambiguous paths and so far it is not even clear what they are (https://gitlab.mpi-sws.org/iris/iris/issues/240).
+-arg -w -arg -ambiguous-paths
 
 theories/lifetime/model/definitions.v
 theories/lifetime/model/faking.v