site/upgrade.md: Add a separate section to known issues during upgrade.
site/upgrade.md: Add a separate section to known issues during upgrade.

To include known bugs preventing stop, mirrored queue sync, etc.

[#154438433]
[#154438645]

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website