Replace uses of libs/json with encoding/json. (#7534)

Where possible, replace uses of the custom JSON library with the standard
library. The custom library treats interface and unnamed lteral types
differently, so this change avoids those even where it would probably be safe
to switch them.

privval/file.go

@@ -3,6 +3,7 @@ package privval
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"os"
@@ -72,7 +73,7 @@ func (pvKey FilePVKey) Save() error {
 
 // FilePVLastSignState stores the mutable part of PrivValidator.
 type FilePVLastSignState struct {
-	Height    int64            `json:"height"`
+	Height    int64            `json:"height,string"`
 	Round     int32            `json:"round"`
 	Step      int8             `json:"step"`
 	Signature []byte           `json:"signature,omitempty"`
@@ -128,7 +129,7 @@ func (lss *FilePVLastSignState) Save() error {
 	if outFile == "" {
 		return errors.New("cannot save FilePVLastSignState: filePath not set")
 	}
-	jsonBytes, err := tmjson.MarshalIndent(lss, "", "  ")
+	jsonBytes, err := json.MarshalIndent(lss, "", "  ")
 	if err != nil {
 		return err
 	}
@@ -215,7 +216,7 @@ func loadFilePV(keyFilePath, stateFilePath string, loadState bool) (*FilePV, err
 		if err != nil {
 			return nil, err
 		}
-		err = tmjson.Unmarshal(stateJSONBytes, &pvState)
+		err = json.Unmarshal(stateJSONBytes, &pvState)
 		if err != nil {
 			return nil, fmt.Errorf("error reading PrivValidator state from %v: %w", stateFilePath, err)
 		}

privval/file_test.go

@@ -3,6 +3,7 @@ package privval
 import (
 	"context"
 	"encoding/base64"
+	"encoding/json"
 	"fmt"
 	"os"
 	"testing"
@@ -105,7 +106,7 @@ func TestUnmarshalValidatorState(t *testing.T) {
 	}`
 
 	val := FilePVLastSignState{}
-	err := tmjson.Unmarshal([]byte(serialized), &val)
+	err := json.Unmarshal([]byte(serialized), &val)
 	require.NoError(t, err)
 
 	// make sure the values match
@@ -114,7 +115,7 @@ func TestUnmarshalValidatorState(t *testing.T) {
 	assert.EqualValues(t, val.Step, 1)
 
 	// export it and make sure it is the same
-	out, err := tmjson.Marshal(val)
+	out, err := json.Marshal(val)
 	require.NoError(t, err)
 	assert.JSONEq(t, serialized, string(out))
 }