Reapply rules for tier-2 for E2E on GDK

What does this MR do and why?

Reapply rules for tier-2 for E2E on GDK

This is a revert of !156896 (merged) so it reapplies it.

The first commit was exactly the same. The second commit should fix the issue: 25c7af5d

Make it equal to tier-1 if there's no tier labels

This should ensure that we don't run all the jobs when the merge request was just created without any tier labels.

Validation

Tested in:

• !157340 (closed) replicating https://gitlab.com/gitlab-org/gitlab/-/pipelines/1340254357
• !157348 (closed) replicating https://gitlab.com/gitlab-org/gitlab/-/pipelines/1340252857

Related to #459977 (closed)