ci: exclude cruft from code coverage

1. Exclude generated code.
2. Exclude generated actors, we mostly test against the latest version.

.codecov.yml

@@ -1,3 +1,18 @@
+ignore:
+  # Auto generated
+  - "^.*_gen.go$"
+  - "^.*/mock_full.go$"
+  # Old actors.
+  - "^chain/actors/builtin/[^/]*/(message|state|v)[0-4]\\.go$" # We test the latest version only.
+  # Tests
+  - "api/test/**"
+  - "conformance/**"
+  # Generators
+  - "gen/**"
+  - "chain/actors/agen/**"
+  # Non-critical utilities
+  - "api/docgen/**"
+  - "api/docgen-openrpc/**"
 coverage:
   status:
     patch: off