[Internal] Reinstate back-compatibility test with older external validators

[bob80905]

Due to an internal change, the step to test back-compatibility with older external validators was removed. This is because it was broken after #7392 went in.
We need to make sure we can bring back this testing, specifically after the change to support the external validator override mechanism goes in. This issue simply tracks that we have some pipeline that tests older external validators, and that the older validators are actually used.