refactor: Update GetSigners to return []string (#9418)

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## Description

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closes: #9239 

---

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- [ ] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/structure.md).
- [ ] Wrote unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
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testutil/testdata/tx.go

@@ -62,22 +62,12 @@ func (msg *TestMsg) GetSignBytes() []byte {
 	}
 	return sdk.MustSortJSON(bz)
 }
-func (msg *TestMsg) GetSigners() []sdk.AccAddress {
-	addrs := make([]sdk.AccAddress, len(msg.Signers))
-	for i, in := range msg.Signers {
-		addr, err := sdk.AccAddressFromBech32(in)
-		if err != nil {
-			panic(err)
-		}
-
-		addrs[i] = addr
-	}
-
-	return addrs
+func (msg *TestMsg) GetSigners() []string {
+	return msg.Signers
 }
 func (msg *TestMsg) ValidateBasic() error { return nil }
 
 var _ sdk.Msg = &MsgCreateDog{}
 
-func (msg *MsgCreateDog) GetSigners() []sdk.AccAddress { return []sdk.AccAddress{} }
-func (msg *MsgCreateDog) ValidateBasic() error         { return nil }
+func (msg *MsgCreateDog) GetSigners() []string { return []string{} }
+func (msg *MsgCreateDog) ValidateBasic() error { return nil }