Extend release notes tool for Serverless

[Ikuni17]

The docs team (cc @benironside @KOTungseth @nastasha-solomon) would like the elastic/kibana-release-notes project extended to support Serverless release notes. See this Google Doc for the full proposal.

To summarize Slack discussions outside of the doc:

• Need to search between the previous serverless release commit and the current release commit to generate notes
• PRs will possibly need to be labeled to distinguish Serverless specific changes and release notes
• Security solution will be the test project before adding release notes to all project types

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)