Replace e2e:test-on-gdk to use e2e:test-on-cng on Tier 3 backport pipelines to run instance scenarios

Problem

e2e:test-on-cng is generally a drop-in replacement for e2e:test-on-gdk. The latter suite is slower and more error prone.

Proposal

Replace e2e:test-on-gdk with e2e:test-on-cng in tier-3 backport MRs