Upgrade to go-amino v0.15.0

[alexanderbez]

• 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.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry: clog add [section] [stanza] [message]

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[alessio]

Please run go mod tidy too and check if go.{mod,sum} get changed - they shouldn't.

[alexanderbez]

@alessio this PR is go mod tidy. That's all I ran apart from updating the dep.

[codecov]

Codecov Report

Merging #4342 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4342      +/-   ##
==========================================
+ Coverage   59.37%   59.38%   +0.01%     
==========================================
  Files         214      214              
  Lines       14533    14533              
==========================================
+ Hits         8629     8631       +2     
+ Misses       5266     5264       -2     
  Partials      638      638