Update the admin user for GET Release Environment QA tests

What does this MR do and why?

Updates the variables used to specify the admin user and password to be used by QA tests in the GET release environments within the CI/CD pipelines. This change is being made to reduce test flakiness caused by authentication failures. These failures are a result of a locked-down default user account with too many login attempts.

References

Ref https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/21373

The referenced issue contains information on the potential approaches considered.

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