migrate db to v2: drop experiments & enrollments w/feature issues or other missing required fields (#4078)

CHANGES_UNRELEASED.md

@@ -13,4 +13,14 @@
 
  - Android and iOS `Branch` objects no longer have access to a `FeatureConfig` object.
 
+### ⚠️ Breaking changes ⚠️
+- The experiment database will be migrating from version 1 to version 2 on
+  first run.  *Various kinds of incorrectly specified feature and featureId
+  related fields will be detected, and any related experiments & enrollments
+  will be discarded.  Experiments & enrollments will also be discarded if they
+  are missing other required fields (eg schemaVersion).  If there is an error
+  during the database upgrade, the database will be wiped, since losing
+  existing enrollments is still less bad than having the database in an unknown
+  inconsistent state.*
+
 [Full Changelog](https://github.com/mozilla/application-services/compare/v76.0.1...main)