Move codecov.yml into .github directory and increase coverage target to 90 percent (#998)

* Move codecov configuration file into .github directory

One less configuration file to see in the top level directory.
See https://docs.codecov.io/docs/codecov-yaml#can-i-name-the-file-codecovyml

* Increase code coverage target to 90%

Author: weiji14

.codecov.yml renamed to .github/codecov.yml

@@ -6,7 +6,7 @@ coverage:
   status:
     patch:
       default:
-        target: '80'
+        target: '90'
         if_no_uploads: error
         if_not_found: success
         if_ci_failed: failure