Include a disclaimer about resources with label maistra.io in Upgrade chapter

In the chapter "Enabling sidecar injection" [1], there is a disclaimer about not labeling user resources with maistra.io.

~~~
maistra.io/ labels and annotations should never be included in user-created resources, because they indicate that the resources are generated and managed by the Operator. If you are copying content from an Operator-generated resource when creating your own resources, do not include labels or annotations that start with maistra.io/ or your resource will be overwritten or deleted by the Operator during the next reconciliation.
~~~

This is specially critical when user is performing a OSSM operator update, for instance from OSSM 2.0 to OSSM 2.1. In this case, if there are resources labelled with maistra.io, those will be deleted. Due to this, it makes sense to include a disclaimer explaining this situation to the user in the chapter "Upgrading Red Hat OpenShift Service Mesh from version 2.0 to version 2.1" [2].

[1] https://docs.openshift.com/container-platform/4.9/service_mesh/v2x/prepare-to-deploy-applications-ossm.html

[2] https://docs.openshift.com/container-platform/4.9/service_mesh/v2x/upgrading-ossm-to-2-1.html