Support variable truncation syntax on GitLab

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Problem

Today customers cannot truncate variables names leading to errors being raised while integrating with 3rd party solutions as explained in &10431.

Proposal

As proposed in #24659 (comment 1350635985) we should follow the bash convention and adopt similar syntax

• Add support for variable truncation in GitLab

${CI_COMMIT_REF_SLUG:0:20}