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