Provide Release Description from Tag in CI/CD variable

What does this MR do and why?

Adds CI variable CI_RELEASE_DESCRIPTION.

Related to #22831 (closed) where we discussed to introduce variables:

• CI_COMMIT_TAG_MESSAGE via !98349 (merged)
• CI_RELEASE_DESCRIPTION introduced in this MR from the proposal #22831 (comment 1106741984)

Documentation MR will be covered separately.

Screenshots or screen recordings

ci_variable_release_description

How to set up and validate locally

image: alpine:latest

stages:
  - test

prod:
  stage: test
  script:
    - echo "Release description is $CI_RELEASE_DESCRIPTION"

• Use the above simple CI configuration to a test project in local and create a release and run pipeline for the corresponding release tag.
• The job logs should include the variable message being printed when echoed as shown in video above.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #22831 (closed)