springboot Update layered jar tests to use snapshots that don't require Apache's snapshot repository

About a week ago we saw 503 responses and then very slow responses from https://repository.apache.org. Today, the SSL certificate has expired:

$ openssl s_client -showcerts -connect repository.apache.org:443                                          
CONNECTED(00000005)
depth=1 C = SE, O = AddTrust AB, OU = AddTrust External TTP Network, CN = AddTrust External CA Root
verify error:num=10:certificate has expired
notAfter=May 30 10:48:38 2020 GMT
verify return:0
depth=1 C = SE, O = AddTrust AB, OU = AddTrust External TTP Network, CN = AddTrust External CA Root
verify error:num=10:certificate has expired
notAfter=May 30 10:48:38 2020 GMT
verify return:0
depth=3 C = SE, O = AddTrust AB, OU = AddTrust External TTP Network, CN = AddTrust External CA Root
verify error:num=10:certificate has expired
notAfter=May 30 10:48:38 2020 GMT
verify return:0

We should update our tests to use snapshots from a different repository so that, hopefully, our build is more stable. Something from https://repo.spring.io, perhaps. It would then be on infrastructure that's somewhat within our control.