add release workflow

[nobodyiam]

Motivation:

Add a release workflow so that it's easy to deploy artifacts to maven central repository.

Modification:

1. upgrade maven-javadoc-plugin to use the configuration doclint
2. add --pinentry-mode option to maven-gpg-plugin
3. add a release workflow so that we could use github action to release the artifacts
   • The workflow uses java 8 to deploy the registry clients as the maven command provided in action/setup-java doesn't support java 6 anymore.
   • We need to determine whether it's ok to drop java 6 support.

Result:

Fixes #327

Sample workflow: https://github.com/nobodyiam/sofa-registry/actions/workflows/release.yml

And the uploaded artifacts could be found in the staging repo:

[codecov]

Codecov Report

Merging #331 (9cb0cff) into master (808b7dc) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #331      +/-   ##
============================================
- Coverage     76.54%   76.52%   -0.02%     
  Complexity       48       48              
============================================
  Files           495      495              
  Lines         18488    18488              
  Branches       1802     1802              
============================================
- Hits          14151    14148       -3     
- Misses         3525     3526       +1     
- Partials        812      814       +2     

see 4 files with indirect coverage changes

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