v2 upgrade guide details for XR connection details and upgrade paths

[jbw976]

This PR updates the v2 upgrade guide with a couple important details I noticed with @ytsarev today:

• Adds an important note box about the necessity to upgrade one minor version at a time. We already have this on the general upgrade guide, but having it in the v2 specific guide for more discoverability will be nice.
• Includes XR connection details breaking change information, similar to how it was added to the back compat section of the what's new in v2 page

These changes are in master and have been synced to all v2.x docs.

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[lsviben]

LGTM, thanks @jbw976