Draft: Pass QA_DEFAULT_BRANCH=main to QA pipelines
This tells the QA framework that the instance uses main as the default branch name.
Part of gitlab-org/gitlab-qa#609 (closed)
This should not be merged until gitlab-org/gitlab!48967 (merged) and gitlab-org/gitlab-qa!596 (merged) are merged and the default branch for Staging has been changed to main.
Edited by Mark Lapierre