Set threshold to 0.03% for code coverage.

We want a little bit of wiggle room for code coverage.
Some legitimate commits that remove code from the system
appear to reduce overall code coverage.

.codecov.yml

@@ -5,5 +5,5 @@ coverage:
     project:
       default:
         target: auto
-        threshold: null
+        threshold: 0.03
         base: auto