Add 5MB dynamic pipeline YAML artifact limit to gitlab.com table

What does this MR do?

Makes specific mention of the 5MB limit for artifact files containing dynamic/child pipeline YAML. Avoids confusion with general artifact size limit which is much larger. Should help avoid support calls relating to pipelines failing when this limit is breached.

Also links existing note about this limit to the open issue covering ways to make the limit larger/configurable.

Related issues

#249140

Author's checklist

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These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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