Merge request description CI variable

What does this MR do and why?

It adds a new predefined variable for the merge request description for CI scripts. This can be as example used for the gitlab release CLI tool to use the MR description as release description.

Fixes #27598 (closed)

Screenshots or screen recordings

How to set up and validate locally

Used GDK and installed a runner locally to ensure the variable is correctly passed to the CI and available.

MR acceptance checklist

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