refactor: rename GenTx to GenSignedMockTx to avoid confusion with genutil/GenTx (backport #11696)

[mergify]

This is an automatic backport of pull request #11696 done by Mergify.
Cherry-pick of 1a9654f has failed:

On branch mergify/bp/release/v0.44.x/pr-11696
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Changes to be committed:
	modified:   simapp/helpers/test_helpers.go
	modified:   simapp/test_helpers.go
	modified:   x/authz/simulation/operations.go
	modified:   x/bank/simulation/operations.go
	modified:   x/genutil/gentx_test.go
	modified:   x/gov/simulation/operations.go
	modified:   x/simulation/util.go
	modified:   x/slashing/simulation/operations.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   CHANGELOG.md
	deleted by us:   x/group/simulation/operations.go
	deleted by us:   x/nft/simulation/operations.go

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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[alexanderbez]

This is actually a breaking change. Closing.