Refine MongoDB upgrade note (#23852)

* Fix link to MongoDB 6.0 upgrade docs

* Mention option to disable preflight check in MongoDB upgrade notes

Author: boosty

UPGRADING.md

@@ -17,10 +17,15 @@ Graylog now requires at least Mongo DB version 7.0. Earlier versions are no long
 In general, MongoDB upgrades must be done from one minor release to the next, going to the latest bug fix version 
 in that release. Please refer to the Mongo DB upgrade documentation for details:
 - [Upgrade tutorial](https://www.mongodb.com/docs/manual/tutorial/upgrade-revision/#std-label-upgrade-to-latest-revision/)
-- [6.0](https://www.mongodb.com/docs/manual/release-notes/6.0-upgrade/)
+- [6.0](https://www.mongodb.com/docs/v6.0/release-notes/6.0/#upgrade-procedures)
 - [7.0](https://www.mongodb.com/docs/manual/release-notes/7.0-upgrade/)
 - [8.0](https://www.mongodb.com/docs/manual/release-notes/8.0-upgrade/)
 
+Graylog users who cannot upgrade to MongoDB >= 7.0 prior to upgrading Graylog to 7.0.x can disable Graylog's preflight 
+check via its configuration setting (`skip_preflight_checks = true`) to let Graylog start regardless of the used MongoDB 
+version. This approach is not recommended though, as Graylog might start using MongoDB 7.0 features over the course of 
+the Graylog 7.x series.
+
 ### Kafka Inputs
 
 The `kafka-clients` library was updated to 4.x which removes support for Kafka