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

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## Description

<!-- Add a description of the changes that this PR introduces and the files that
are the most critical to review.
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closes: #9239 

---

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write a little note why.

- [ ] Targeted PR against correct branch (see [CONTRIBUTING.md](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [ ] 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)
- [ ] Updated relevant documentation (`docs/`) or specification (`x/<module>/spec/`)
- [ ] Added relevant `godoc` [comments](https://blog.golang.org/godoc-documenting-go-code).
- [ ] Added a relevant changelog entry to the `Unreleased` section in `CHANGELOG.md`
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baseapp/baseapp.go

@@ -19,6 +19,7 @@ import (
 	"github.com/cosmos/cosmos-sdk/store/rootmulti"
 	sdk "github.com/cosmos/cosmos-sdk/types"
 	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
+	sdktx "github.com/cosmos/cosmos-sdk/types/tx"
 	"github.com/cosmos/cosmos-sdk/x/auth/migrations/legacytx"
 )
 
@@ -508,7 +509,7 @@ func validateBasicTxMsgs(msgs []sdk.Msg) error {
 	}
 
 	for _, msg := range msgs {
-		err := msg.ValidateBasic()
+		err := sdktx.ValidateMsg(msg)
 		if err != nil {
 			return err
 		}

baseapp/baseapp_test.go

@@ -717,10 +717,10 @@ func (msg msgCounter) String() string { return "TODO" }
 func (msg msgCounter) ProtoMessage()  {}
 
 // Implements Msg
-func (msg msgCounter) Route() string                { return routeMsgCounter }
-func (msg msgCounter) Type() string                 { return "counter1" }
-func (msg msgCounter) GetSignBytes() []byte         { return nil }
-func (msg msgCounter) GetSigners() []sdk.AccAddress { return nil }
+func (msg msgCounter) Route() string        { return routeMsgCounter }
+func (msg msgCounter) Type() string         { return "counter1" }
+func (msg msgCounter) GetSignBytes() []byte { return nil }
+func (msg msgCounter) GetSigners() []string { return nil }
 func (msg msgCounter) ValidateBasic() error {
 	if msg.Counter >= 0 {
 		return nil
@@ -762,10 +762,10 @@ func (msg msgCounter2) String() string { return "TODO" }
 func (msg msgCounter2) ProtoMessage()  {}
 
 // Implements Msg
-func (msg msgCounter2) Route() string                { return routeMsgCounter2 }
-func (msg msgCounter2) Type() string                 { return "counter2" }
-func (msg msgCounter2) GetSignBytes() []byte         { return nil }
-func (msg msgCounter2) GetSigners() []sdk.AccAddress { return nil }
+func (msg msgCounter2) Route() string        { return routeMsgCounter2 }
+func (msg msgCounter2) Type() string         { return "counter2" }
+func (msg msgCounter2) GetSignBytes() []byte { return nil }
+func (msg msgCounter2) GetSigners() []string { return nil }
 func (msg msgCounter2) ValidateBasic() error {
 	if msg.Counter >= 0 {
 		return nil
@@ -779,13 +779,13 @@ type msgKeyValue struct {
 	Value []byte
 }
 
-func (msg msgKeyValue) Reset()                       {}
-func (msg msgKeyValue) String() string               { return "TODO" }
-func (msg msgKeyValue) ProtoMessage()                {}
-func (msg msgKeyValue) Route() string                { return routeMsgKeyValue }
-func (msg msgKeyValue) Type() string                 { return "keyValue" }
-func (msg msgKeyValue) GetSignBytes() []byte         { return nil }
-func (msg msgKeyValue) GetSigners() []sdk.AccAddress { return nil }
+func (msg msgKeyValue) Reset()               {}
+func (msg msgKeyValue) String() string       { return "TODO" }
+func (msg msgKeyValue) ProtoMessage()        {}
+func (msg msgKeyValue) Route() string        { return routeMsgKeyValue }
+func (msg msgKeyValue) Type() string         { return "keyValue" }
+func (msg msgKeyValue) GetSignBytes() []byte { return nil }
+func (msg msgKeyValue) GetSigners() []string { return nil }
 func (msg msgKeyValue) ValidateBasic() error {
 	if msg.Key == nil {
 		return sdkerrors.Wrap(sdkerrors.ErrInvalidRequest, "key cannot be nil")