Release notes script: change output from asciidoc to json

[karenzone]

Update Logstash release notes generator to output json in preparation for consolidated searchable release notes in new docs system.

Suggestion from @jsvd: We can scope and complete the work now, and set a flag not to cut over to json formatting until the time is right for moving forward with the project.

cc:/ @jsvd @roaksoax @gtback

[karenzone]

Closing. Replaced by #16853.