Use project default branch in QA tests

What does this MR do?

Allows tests to be run on an instance where the default branch has been changed.

This does not require setting an environment variable (unlike !48649 (closed)) because the framework will use the project's default branch name.

If the instance does not have a default initial branch name set, projects will currently be created with master as the default. If that default is changed in the source code, the framework will need to be updated. To minimize any interruption while that change is made, the framework will use the value of the QA_DEFAULT_BRANCH environment variable (after gitlab-qa!596 (merged) is merged).

Additional testing in !48726 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content