v2: Stop updating major-version branch #35

Summary

Stop creating/updating the floating v1 major-version branch on release. Consumers should pin by SHA with an exact version tag comment instead.

This is a breaking change for consumers currently pinned to @v1 (the branch), which is why it belongs in a major version bump. Consumers pinned by SHA are unaffected.

Work

• Remove or disable the major-version branch update from the release workflow
• Update documentation to recommend SHA pinning: uses: pantheon-systems/phpcompatibility-action@<sha> # v2.0.0
• Leave existing v1 branch in place — it just stops being updated

Context

• Jira: SITE-5949
• Epic: SITE-5932
• Background: scratchpad PR #331, release-pipeline-actions-plan.md item 1