Allow up to 0.05% drop in code coverage per commit (deephaven#791)

- Previously required each change to increase test code coverage, which is unrealistic for a lot of bug fixes
- Change the threshold to 0.05% so bug fixes can go in without needed to increase the threshold, but bigger features should still trigger it

codecov.yml

@@ -1,3 +1,10 @@
 flag_management:
   default_rules:
     carryforward: true
+coverage:
+  status:
+    project:
+      default:
+        # Allow up to a 0.05% drop in project threshold for codecov
+        # A lot of bug fixes don't change test coverage, and then codecov fails
+        threshold: 0.05%