[Spike] CI: Consider enabling merge trains

Summary

Lots of the Charts pipeline failures on master are related to the long-running Charts environments getting into an undesirable state due to conflicting pipelines.

For example, an older pipeline can be running QA tests that manipulate data in an environment while a newer pipeline is deploying a newer release (#5139 (comment 1678035204)).

This is exacerbated when multiple merge requests are merged in quick succession.

We can address this with comprehensive use of environment locking (resource_group, etc.). However, it may be more easier and more reliable to instead enable merge trains on the project.

Acceptance criteria

• Evaluate merge trains implementation for Charts
• If we choose to enable it, create a follow-up issue for implementation