vendor: bump golang/mock to 1.6.0

Commit e3ed3ba7c9 bumps golang/mock to 1.6.0 in hack/tools.
It makes sense to keep its version in sync, so let's do the same
in the top level go.mod.

Generated by

	./hack/pin-dependency.sh github.com/golang/mock v1.6.0
	./hack/lint-dependencies.sh
	./hack/update-vendor.sh
	./hack/update-internal-modules.sh

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

vendor/github.com/golang/mock/gomock/callset.go

@@ -16,6 +16,7 @@ package gomock
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 )
 
@@ -95,7 +96,7 @@ func (cs callSet) FindMatch(receiver interface{}, method string, args []interfac
 		_, _ = fmt.Fprintf(&callsErrors, "there are no expected calls of the method %q for that receiver", method)
 	}
 
-	return nil, fmt.Errorf(callsErrors.String())
+	return nil, errors.New(callsErrors.String())
 }
 
 // Failures returns the calls that are not satisfied.