docs: fix dead link in UPGRADING.md (backport #23059)

[mergify]

Description

simapp/CHANGELOG.md was removed in PR #23009 , but this file is required in UPGRADING.md.

In this section we describe the changes made in Cosmos SDK' SimApp.
These changes are directly applicable to your application wiring.
Please read this section first, but for an exhaustive list of changes, refer to the CHANGELOG.

Maybe this file was mis-deleted and should be recovered?

Summary by CodeRabbit

• Documentation
  • Updated UPGRADING.md to reflect significant changes in the Cosmos SDK.
  • Removed the x/params module and added the x/protocolpool module.
  • Clarified migration processes and new features, including unordered transactions and expedited proposals.
  • Provided guidance on necessary code adjustments for developers.

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This is an automatic backport of pull request #23059 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2596c7f has failed:

On branch mergify/bp/release/v0.52.x/pr-23059
Your branch is up to date with 'origin/release/v0.52.x'.

You are currently cherry-picking commit 2596c7f6b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   UPGRADING.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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