Bump protobuf lib and fix deprecation warnings (#285)

* Bump github.com/golang/protobuf from 1.4.2 to 1.5.2

Bumps [github.com/golang/protobuf](https://github.com/golang/protobuf) from 1.4.2 to 1.5.2.
- [Release notes](https://github.com/golang/protobuf/releases)
- [Commits](https://github.com/golang/protobuf/compare/v1.4.2...v1.5.2)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
---

* Also pulls along update of golang.google.org/protobuf to v1.26
* Fix deprecation warnings about `ptypes` package

internal/testing/cmd/bankdemo/bank.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"time"
 
-	"github.com/golang/protobuf/ptypes"
 	"github.com/golang/protobuf/ptypes/empty"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
@@ -72,26 +71,29 @@ func (s *bankServer) GetTransactions(req *GetTransactionsRequest, stream Bank_Ge
 
 	var start, end time.Time
 	if req.Start != nil {
-		start, err = ptypes.Timestamp(req.Start)
+		err := req.Start.CheckValid()
 		if err != nil {
 			return err
 		}
+		start = req.Start.AsTime()
 	}
 	if req.End != nil {
-		end, err = ptypes.Timestamp(req.End)
+		err := req.End.CheckValid()
 		if err != nil {
 			return err
 		}
+		end = req.End.AsTime()
 	} else {
 		end = time.Date(9999, 12, 31, 23, 59, 59, 999999999, time.Local)
 	}
 
 	txns := acct.getTransactions()
 	for _, txn := range txns {
-		t, err := ptypes.Timestamp(txn.Date)
+		err := txn.Date.CheckValid()
 		if err != nil {
 			return err
 		}
+		t := txn.Date.AsTime()
 		if (t.After(start) || t.Equal(start)) &&
 			(t.Before(end) || t.Equal(end)) {