Bug 1886830: update staging content signing root certificate to new cas_cur...

Bug 1886830: update staging content signing root certificate to new cas_cur root certificate  a=RyanVM

This certificate is `cas-cur-root-2024-03-12.crt` from bug 1882192 (https://bugzilla.mozilla.org/attachment.cgi?id=9390858), and has been converted to DER with:

$ openssl x509 -in cas-cur-root-2024-03-12.crt -inform PEM -outform DER

Original Revision: https://phabricator.services.mozilla.com/D205356

Differential Revision: https://phabricator.services.mozilla.com/D205774