Use codecov in informational mode

Codecov coverage reporting is somewhat unstable, with numbers changing
back and forth infrequently. This is causing failure indications on CI
runs. To prevent that from happening moving forward, run it in
informational mode [0] instead.

[0]: https://docs.codecov.com/docs/commit-status#informational

.github/.codecov.yml

@@ -0,0 +1,10 @@
+coverage:
+  status:
+    project:
+      default:
+        # We don't gate anything based on potentially flaky code
+        # coverage reports.
+        informational: true
+    patch:
+      default:
+        informational: true