Merge branch 'remove-custom-webhook-headers-flag' into 'master'

Remove custom_webhook_headers feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149910



Merged-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Approved-by: Luke Duncalfe <lduncalfe@gitlab.com>
Approved-by: Evan Read <eread@gitlab.com>
Approved-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Co-authored-by: Niklas <mc.taucher2003@gmail.com>