refactor: move from io/ioutil to io and os packages

The io/ioutil package has been deprecated as of Go 1.16 [1]. This commit
replaces the existing io/ioutil functions with their new definitions in
io and os packages.

Reference: https://golang.org/doc/go1.16#ioutil
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

logtail/logtail.go

@@ -13,7 +13,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"os"
 	"strconv"
@@ -430,7 +429,7 @@ func (l *Logger) upload(ctx context.Context, body []byte, origlen int) (uploaded
 
 	if resp.StatusCode != 200 {
 		uploaded = resp.StatusCode == 400 // the server saved the logs anyway
-		b, _ := ioutil.ReadAll(io.LimitReader(resp.Body, 1<<20))
+		b, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
 		return uploaded, fmt.Errorf("log upload of %d bytes %s failed %d: %q", len(body), compressedNote, resp.StatusCode, b)
 	}
 
@@ -654,7 +653,7 @@ func (l *Logger) Write(buf []byte) (int, error) {
 		return 0, nil
 	}
 	level, buf := parseAndRemoveLogLevel(buf)
-	if l.stderr != nil && l.stderr != ioutil.Discard && int64(level) <= atomic.LoadInt64(&l.stderrLevel) {
+	if l.stderr != nil && l.stderr != io.Discard && int64(level) <= atomic.LoadInt64(&l.stderrLevel) {
 		if buf[len(buf)-1] == '\n' {
 			l.stderr.Write(buf)
 		} else {