changelog - use description field for release bundles

[SylvainJuge]

Support for description field has been added in elastic/docs-builder#3058 in docs-builder.

This will be included in docs-builder 1.2.0 release, which is not available yet.

• add description field in existing changelog bundles (this could be extracted from the content of the release-notes before release notes switch to docs-builder yaml #1023 is merged: https://github.com/elastic/elastic-otel-java/blob/b100c122b668ef2a5366b60ec6f3141914aa5c9a/docs/release-notes/index.md

• modify the .ci/changelog-upstream-update.sh script to remove adding the description for the upstream update

• modify the bundle creation at release time added in update release changelog with docs-builder #1047 to add a description field with the upstream dependencies.

• modify (and rename) the gradle tasks that generates one-liner markdown to be used to generate multi-line markdown string to be provided at the release bundle creation time. Reusing the existing printUpstreamDependenciesMarkdown task would be appropriate to generate the markdown for bundle description.

• Examples how to provide a multi-line markdown description are provided in Add changelog bundle description docs-builder#3058 pr description.

• at release time, add release-date field with the date at the time of release.
  Update: this has been added in Add release_dates changelog config SylvainJuge/elastic-otel-java#3 which is included in [changelog] add release dates + descriptions to existing releases bundles #1048