chore: improve UGPRADING.md

[julienrbrt]

Description

Follow-up of #15211

• Add changelog
• Fix proto lint
• Reword UPGRADING.md

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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 type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation 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...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[tac0turtle]

nit: can we combine the two migration to comet bft sections into one?

[julienrbrt]

Part 2 is unreleased and Part 1 is under v0.47, so I don't think so?

[tac0turtle]

oh i thought what is on main is also used in 0.47 so it seemed to be overlapping.

Oh these are items we changed in our repo. Kind of confusing

[julienrbrt]

I don't really follow. There were some thing repetitive from the other PR, but I've removed it here.
There is indeed some categories that are the same between unreleased and v0.47, but this is because we've changed more things after v0.47 had its scope fixed. So this is why it is under unreleased.

[tac0turtle]

it was mainly confusing to see part 1 and part 2 but they are not in the same release and have two different scopes.

[julienrbrt]

We can reword. As it was both related I thought it was clear. I am not sure how to name the "Part 2" otherwise, as I feel like it's still clearly related.