Doc: Update release notes scripts to generate content in MD instead of ASCIIDOC

[karenzone]

Related: https://github.com/elastic/docs-projects/issues/276

Current release notes generator is available as a GitHub action: https://github.com/elastic/logstash/actions/workflows/gen_release_notes.yml

Requirements

• Add ability to generate release notes in MD format for 9.0.0 and later
• Maintain ability to generate release notes in ADOC format for 8.x and earlier

Resources

https://elastic.github.io/docs-builder/migration/guide/index.html