add note about request_input in upgrade guide

[thejohncotton]

Description

Adding a note about how requesting input has changed to the upgrade guide.

Motivation and Context

The change in requesting input events for a scene from implicit in 0.10 to explicit in 0.11 upgrade was not 100% clear from the upgrade guide. Based on the discussion in slack I think this may be a small improvement to help folks during the upgrade. I'm not 100% sure where else it may be useful to add documentation for this but I am happy to add some with the appropriate guidance.

Types of changes

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist

• [x] Check other PRs and make sure that the changes are not done yet.
• [x] The PR title is no longer than 64 characters.

[crertel]

I think this got me, thank you for the PR!