Armand pointed out this should follow the same release-branch transition pattern used for v6 in #13176 and #13373, rather than removing main updates entirely or renaming the generated v7 PR branches.

During this phase, the docs repository still has two relevant branches:
- docs main is still the current v6 documentation branch.
- docs v7 is the upcoming v7 documentation branch.

The Astro framework repository has v7 on main now, and also has a 6-legacy branch for any last-minute v6 fixes. That means the nightly generated reference jobs should keep two matrix entries:
- SOURCE_BRANCH: 6-legacy, TARGET_BRANCH: main, so current docs do not receive v7-generated reference content but can still pick up v6 fixes.
- SOURCE_BRANCH: main, TARGET_BRANCH: v7, so the v7 docs branch receives generated reference updates from the framework main branch.

This mirrors #13176, where the previous docs main branch sourced from 5-legacy while the v6 docs branch sourced from framework main. The later post-merge cleanup is the separate #13373-style step: switch docs main back to source main and comment out the version-branch entry after v7 itself is merged.