Docs: specify regexp language used for `only`/`except` mechanism in `.gitlab-ci.yaml`

What does this MR do?

Specifies the regexp language used in .gitlab-ci.yaml's only/except mechanism.

Author's checklist

• Apply the correct labels and milestone: I do not know which Milestone should be set
• Crosslink the document from the higher-level index: Not applicable
• Crosslink the document from other subject-related docs: Not applicable
• Feature moving tiers? Make sure the change is also reflected in features.yml: I am not competent on that matter
• Correctly apply the product badges and tiers: I am not competent on that matter
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails - I am not competent on that matter

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards