Add _old-network: false for bson & mongoDB

Packages "bson" and "mongoDB" are compatible with newer network, but
require a flag for it.

build-constraints.yaml

@@ -3114,8 +3114,8 @@ packages:
         - open-witness < 0 # MonadFail
 
     "Victor Denisov <denisovenator@gmail.com> @VictorDenisov":
-        - mongoDB < 0 # via network-3.1.1.0
-        - bson < 0 # via network-3.1.1.0
+        - mongoDB
+        - bson
 
     "Alexis King <lexi.lambda@gmail.com> @lexi-lambda":
         - freer-simple
@@ -4760,6 +4760,12 @@ package-flags:
     hsdev:
         docs: false
 
+    bson:
+        _old-network: false
+
+    mongoDB:
+        _old-network: false
+
 # end of package-flags
 
 # Special configure options for individual packages