Increase code coverage threshold to 75%

[Shiwei Zhang]

As we are putting more efforts into the quality of the oras-go library, I suggest increasing threshold of the code coverage of the unit test to 75% from 70% and enforce code coverage for PR patches.

oras-go/.github/.codecov.yml

Lines 14 to 19 in 5073458

 coverage: 
   status: 
     project: 
       default: 
         target: 70% 
         if_ci_failed: error 

By increasing the code coverage, we aim to make oras-go more stable with better quality.