Give sqlite compatibility warning a distinct warning code

This prevents the fallback warning from interfering with the test wagtail.search.tests.test_indexed_class.TestSearchFields.test_checking_core_page_fields_are_indexed

wagtail/search/apps.py

@@ -28,5 +28,5 @@ class WagtailSearchAppConfig(AppConfig):
         if connection.vendor == 'sqlite':
             import sqlite3
             if sqlite3.sqlite_version_info < (3, 19, 0):
-                return [Warning('Your SQLite version is older than 3.19.0. A fallback search backend will be used instead.', hint='Upgrade your SQLite version to at least 3.19.0', id='wagtailsearch.W001', obj=WagtailSearchAppConfig)]
+                return [Warning('Your SQLite version is older than 3.19.0. A fallback search backend will be used instead.', hint='Upgrade your SQLite version to at least 3.19.0', id='wagtailsearch.W002', obj=WagtailSearchAppConfig)]
         return []