Remove the manual process for cherry-picking security MRs

Mawreen Dela Cruzrequested to merge
mdc/cherrypick-security-mrs into master

This MR removes the manual process for cherry-picking security MRs since the process is now automated similar to canonical MRs following the implementation of gitlab-com/gl-infra/delivery#20810 (closed). This is a follow-up to the MR.

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