Fix multisig LegacyAminoPubKey Amino marshaling

[amaury1093]

Description

revert: #8794 (that was a temporary workaround)
closes: #8776

PR description in crypto/keys/multisig/amino.go comments

To test, I did:

• take the JSON file by Akhil in migrate command failed with error cannot unmarshal string into Go value of type uint32 #8776
• temporarily change the default bech32 prefix from "cosmos" to "akash" to match the JSON file
• run simd migrate v0.40 <json_file>
• make sure the account_number:"90" has correct NON-EMPTY multisig field.

---

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• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[amaury1093]

Would love to have @anilcse or @akhilkumarpilli try this manually.

[akhilkumarpilli]

Tested ACK. Thanks @AmauryM

[alessio]

Tests are failing

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[codecov]

Codecov Report

Merging #8841 (f46993f) into master (c6af0ed) will increase coverage by 0.01%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master    #8841      +/-   ##
==========================================
+ Coverage   59.15%   59.17%   +0.01%     
==========================================
  Files         570      571       +1     
  Lines       31770    31800      +30     
==========================================
+ Hits        18793    18817      +24     
- Misses      10776    10780       +4     
- Partials     2201     2203       +2     

Impacted Files	Coverage Δ
crypto/keys/multisig/codec.go	100.00% <ø> (ø)
server/test_helpers.go	0.00% <0.00%> (ø)
crypto/keys/multisig/amino.go	70.37% <70.37%> (ø)
x/auth/client/cli/tx_multisign.go	64.88% <71.42%> (+0.04%)	⬆️
x/auth/client/cli/tx_sign.go	74.59% <100.00%> (ø)
x/auth/legacy/legacytx/stdtx.go	80.85% <100.00%> (ø)
x/auth/legacy/v040/migrate.go	90.16% <100.00%> (+2.86%)	⬆️
x/genutil/legacy/v038/migrate.go	82.75% <100.00%> (+0.61%)	⬆️

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