BB-20108: Enable language optimization for websearch indexes by default (#30063)

updated links in CHANGELOG.md
updated documentation to mention that search reindexation is required after upgrade
moved upgrade notes from packages to the application level and removed UPGRADE.md in packages

Author: anyt

Diff for: CHANGELOG.md

@@ -0,0 +1,4 @@
+The upgrade instructions are available at [Oro documentation website](https://doc.oroinc.com/master/backend/setup/upgrade-to-new-version/).
+
+The current file describes significant changes in the code that may affect the upgrade of your customizations.
+