uSync: Umbraco 17.7.0 readiness review

Repo: KevinJump/uSync

Reviewed the upcoming Umbraco 17.7.0 release (160 commits since 17.6.2) against uSync's current behaviour
Found three things that need work: the MovePropertyType(alias, null) core bug (#1009) is fixed upstream, so our workaround's test needs rewriting; migration plan cache-rebuild flag no longer leaks into uSync_FirstBoot on upgrade paths, which may leave the published cache stale; culture code casing is now normalized on save, which could shift export output and needs checking
Opened KevinJump/uSync#1067 with checkpoints for all three, plus a table of ten other reviewed-but-fine changes