Fix multisig LegacyAminoPubKey Amino marshaling (bp #8841)

[mergify]

This is an automatic backport of pull request #8841 done by Mergify.

closes: #8776

Cherry-pick of d4d27e1 has failed:

On branch mergify/bp/release/v0.42.x/pr-8841
Your branch is up to date with 'origin/release/v0.42.x'.

You are currently cherry-picking commit d4d27e1c0.
  (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:
	new file:   crypto/keys/multisig/amino.go
	modified:   crypto/keys/multisig/codec.go
	modified:   crypto/keys/multisig/multisig_test.go
	modified:   x/auth/legacy/legacytx/stdtx.go
	modified:   x/genutil/legacy/v038/migrate.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   x/auth/legacy/v040/migrate.go

To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.io/

[orijbot]

Benchmark finished. See the result: https://github.orijtech.com/benchmark/result?id=0d0fccfae5f74809821d0d96155ece66

[orijbot]

Benchmark beginning. Status page: https://github.orijtech.com/benchmark/status?commit=c292fb458cf7152658dbdbab95852b27371ac686

[orijbot]

Benchmark finished. See the result: https://github.orijtech.com/benchmark/result?id=5f893123711c4d0e9cd7ff6c6a910bfb

[orijbot]

Benchmark beginning. Status page: https://github.orijtech.com/benchmark/status?commit=1e17860e8929afc0f6c3c6980258ffea2d06ae82