PLT-7223: Update "Database schema cannot be upgraded" error message to indicate expected version (#7079)

* PLT-7223: Updated error message to include relevant schema versions

* PLT-7223: Removed unit test. It poisons database state, and there's no recovery mechanism, because the code under test causes an ungraceful exit, so we can't fix the database in a defer statement. This causes the next test execution or server start to fail.

* PLT-7223: Incorporated @crspeller's suggestions into error message string

i18n/en.json

@@ -5261,7 +5261,7 @@
   },
   {
     "id": "store.sql.schema_version.critical",
-    "translation": "The database schema version of %v cannot be upgraded.  You must not skip a version."
+    "translation": "Database schema version %v is no longer supported. This Mattermost server supports automatic upgrades from schema version %v through schema version %v. Downgrades are not supported. Please manually upgrade to at least version %v before continuing"
   },
   {
     "id": "store.sql.short_ciphertext",