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 Canary has been changed to main.

See also gitlab-com/gl-infra/production#3065 (closed)