Merge branch 'delete-old-feature-flags-pipeline_stage_set_last_modified' into 'master'

Delete the `pipeline_stage_set_last_modified` feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182100



Merged-by: Rajendra Kadam <rkadam@gitlab.com>
Approved-by: Paulo Barros <pbarros@gitlab.com>
Approved-by: Rajendra Kadam <rkadam@gitlab.com>
Reviewed-by: Rajendra Kadam <rkadam@gitlab.com>
Reviewed-by: Paulo Barros <pbarros@gitlab.com>
Co-authored-by: Siddharth Dungarwal <sdungarwal@gitlab.com>