codecov config: enable carryforward (#17454)

With flags, and potentially not all coverage info will be submitted on
every commit, it seems that carryforward is necessary.

https://docs.codecov.com/docs/carryforward-flags

codecov.yml

@@ -37,8 +37,10 @@ flags:
       - enterprise/backend
       - shared/src
       - src/metabase
+    carryforward: true
 
   front-end:
     paths:
       - enterprise/frontend
-      - frontend
\ No newline at end of file
+      - frontend
+    carryforward: true