docs: add UPGRADING.md template

[MSalopek]

Description

Add a template for upgrade instructions.

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Author Checklist

All items are required. Please add a note to the item if the item is not applicable and please add links to any relevant follow up issues.

I have...

• [ ] included the correct docs: prefix in the PR title
• [ ] targeted the correct branch (see PR Targeting)
• [ ] provided a link to the relevant issue or specification
• [ ] reviewed "Files changed" and left comments if necessary
• [ ] confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add your handle next to the items reviewed if you only reviewed selected items.

I have...

• [x] Confirmed the correct docs: prefix in the PR title
• [x] Confirmed all author checklist items have been addressed
• [x] Confirmed that this PR only changes documentation
• [x] Reviewed content for consistency
• [x] Reviewed content for thoroughness
• [x] Reviewed content for spelling and grammar
• [x] Tested instructions (if applicable)