codecov: formalize coverage settings (#6031)

Co-authored-by: Sourcegraph Bot <campaigns@sourcegraph.com>

codecov.yml

@@ -1,2 +1,9 @@
+coverage:
+  range: "60...95"
+  status:
+    project:
+      default:
+        threshold: 1%
+
 comment:
   layout: 'diff, files'