[PM-24284] - milestone 3 (#6543)

* new feature flag

* first pass at changes

* safeguard against billing-pricing not being deployed yet

* handle families pre migration plan

* wrong stripe id

* tests

* unit tests

src/Core/Utilities/StaticStore.cs

@@ -137,6 +137,7 @@ public static class StaticStore
             new Teams2019Plan(true),
             new Teams2019Plan(false),
             new Families2019Plan(),
+            new Families2025Plan()
         }.ToImmutableList();
     }