[#638] Update to Go 1.16

Changes:

  * replace `iotuil` elements with the ones from `os` package;
  * replace `os.Filemode` with `fs.FileMode`;
  * use `signal.NotifyContext` instead of `NewGracefulContext` (removed).

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/util/locode/table/csv/opts.go

@@ -1,14 +1,14 @@
 package csvlocode
 
 import (
-	"os"
+	"io/fs"
 )
 
 // Option sets an optional parameter of Table.
 type Option func(*options)
 
 type options struct {
-	mode os.FileMode
+	mode fs.FileMode
 
 	extraPaths []string
 }