Close old open MRs in Secure test projects

Problem to solve

The Secure test projects cumulate about 300 open MRs, which makes it harder to see which MRs need to be addressed.

This is a one-shot cleanup. We might want to improve the docs, and remind developers they should remove close MRs they've used for testing purposes.

Proposal

close all open MRs that are more than 1 month old, and make sure their source branches are removed at the same time

/cc @willmeek