Remove QA_DEFAULT_BRANCH from the E2E tests

When the default branch switched from master to main there was testing executed across both branches and we needed a way to change the default branch for the entire test run. We no longer need this option, and it has caused issues in the past by creating a situation where a test can pass in MRs and fail in staging.

Example: QA_DEFAULT_BRANCH is set to master in staging - this leads to some confusing behaviour when creating branches in tests: https://gitlab.com/gitlab-org/gitlab/-/issues/525001 failed in staging whereas it passed in MRs.

We should remove QA_DEFAULT_BRANCH and keep main as the default so it is consistent in all pipelines/locally. Engineers writing tests can set different branches within tests if required.

Details

The following projects have QA_DEFAULT_BRANCH set:

Project	Set?	Value
https://gitlab.com/gitlab-org/gitlab/-/settings/ci_cd#js-cicd-variables-settings	no	✅
https://ops.gitlab.net/gitlab-org/quality/production/-/settings/ci_cd#js-cicd-variables-settings	no	✅
https://ops.gitlab.net/gitlab-org/quality/canary/-/settings/ci_cd#js-cicd-variables-settings	no	✅
https://ops.gitlab.net/gitlab-org/quality/staging-ref/-/settings/ci_cd#js-cicd-variables-settings	no	✅
https://ops.gitlab.net/gitlab-org/quality/staging/-/settings/ci_cd#js-cicd-variables-settings	yes - master	❌
https://ops.gitlab.net/gitlab-org/quality/staging-canary/-/settings/ci_cd#js-cicd-variables-settings	yes - master	❌

Steps to complete

• 1. Update the QA_DEFAULT_BRANCH to main in above projects that have QA_DEFAULT_BRANCH set, ensure test run passes.
• 1. Update codebase to not refer to QA_DEFAULT_BRANCH gitlab-org/gitlab!203343 (merged)
• 1. Remove any CI/CD variables for QA_DEFAULT_BRANCH
• 1. Remove default_branch from terraform EP infra project MR: https://gitlab.com/gitlab-org/quality/engineering-productivity-infrastructure/-/merge_requests/1054