mount: fix Go 1.20 errorlint warning

Since Go 1.20,

> [t]he fmt.Errorf function now supports multiple occurrences of the %w
> format verb, which will cause it to return an error that wraps all of
> those error operands.

This change, together with newer errorlint, causes the following
warning:

> mount_unix.go:74:56: non-wrapping format verb for fmt.Errorf. Use `%w` to format errors (errorlint)
> 					return fmt.Errorf("%w (possible cause: %s)", err, suberr)
>					                                                  ^

Since Go < 1.20 is no longer supported, let's wrap both errors,
and set go version to 1.20 in go.mod

Fixes: 487129d
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

mount/go.mod

@@ -1,6 +1,6 @@
 module github.com/moby/sys/mount
 
-go 1.16
+go 1.20
 
 require (
 	github.com/moby/sys/mountinfo v0.6.2

mount/mount_unix.go

@@ -71,7 +71,7 @@ func RecursiveUnmount(target string) error {
 		if err != nil {
 			if i == lastMount {
 				if suberr != nil {
-					return fmt.Errorf("%w (possible cause: %s)", err, suberr)
+					return fmt.Errorf("%w (possible cause: %w)", err, suberr)
 				}
 				return err
 			}