docs: add chain upgrade guide v0.43 (backport #9567)

[mergify]

This is an automatic backport of pull request #9567 done by Mergify.
Cherry-pick of 6098d7e has failed:

On branch mergify/bp/release/v0.43.x/pr-9567
Your branch is up to date with 'origin/release/v0.43.x'.

You are currently cherry-picking commit 6098d7ed5.
  (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)

Changes to be committed:
	modified:   docs/building-modules/upgrade.md
	modified:   docs/core/upgrade.md
	deleted:    docs/migrations/app_and_modules.md
	deleted:    docs/migrations/chain-upgrade-guide-040.md
	new file:   docs/migrations/chain-upgrade-guide-043.md
	deleted:    docs/migrations/keyring.md
	modified:   docs/migrations/rest.md

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

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

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