Remove ci_matrix_expressions feature flag

What does this MR do and why?

Enables matrix expressions feature by default, allowing the use of $[[ matrix.IDENTIFIER ]] syntax in job dependencies for dynamic 1:1 mappings between parallel:matrix jobs.

Feature issue: Support dynamic job dependencies in `needs:para... (#423553)

Rollout issue: [FF] Rollout ci_matrix_expressions gitlab_com_d... (#571105)

Changelog: added