Update links to hidden variables

Migrated issue

Reported by: drunken monkey

Related to !229 (merged)

Problem/Motivation

In #3343522: Hide variables that should not be overridden, the variables that should not be overridden by individual projects/pipelines were moved to the include.drupalci.hidden-variables.yml file. However, the descriptions of the three variables that use semantic labels (which are now defined in hidden-variables.yml) still reference the old file.

Steps to reproduce

Run a custom pipeline and try to find the semantic labels by following the displayed link.

Proposed resolution

Fix the referenced URL.

Remaining tasks

• Review and merge the MR.

Related issue: Issue #3343522