feat: use writer directly in stdLogger.

[kvii]

Description (what this PR does / why we need it):

log.stdLogger use log.Logger internally, and disable all options such as prefix or log flags. log.stdLogger use l.log like a io.Writer. So I removed l.log, use io.Writer directly.

Code "stdLogger use log.Logger with no options":

kratos/log/std.go

Lines 20 to 21 in 08300d8

	return &stdLogger{
	log: log.New(w, "", 0),

Code "stdLogger not use any format features of log.Logger":

kratos/log/std.go

Lines 39 to 43 in 08300d8

	buf.WriteString(level.String())
	for i := 0; i < len(keyvals); i += 2 {
	_, _ = fmt.Fprintf(buf, " %s=%v", keyvals[i], keyvals[i+1])
	}
	_ = l.log.Output(4, buf.String()) //nolint:gomnd

Which issue(s) this PR fixes (resolves / be part of):

Other special notes for the reviewers:

I did not add test for stdLogger.Close, because there is no way to call Close outside.

[kvii]

A benchmark for it.

• new code

goos: darwin
goarch: arm64
pkg: github.com/go-kratos/kratos/v2/log
BenchmarkNew-10    	13301796	        75.79 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/go-kratos/kratos/v2/log	1.344s

• old code

goos: darwin
goarch: arm64
pkg: github.com/go-kratos/kratos/v2/log
BenchmarkOld-10    	 6942644	       160.6 ns/op	      32 B/op	       1 allocs/op
PASS
ok  	github.com/go-kratos/kratos/v2/log	1.525s

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.64%. Comparing base (05b23fb) to head (116b43d).

❗ Current head 116b43d differs from pull request most recent head 6c6a554. Consider uploading reports for the commit 6c6a554 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3121      +/-   ##
==========================================
+ Coverage   81.62%   81.64%   +0.01%     
==========================================
  Files          91       91              
  Lines        4163     4167       +4     
==========================================
+ Hits         3398     3402       +4     
  Misses        587      587              
  Partials      178      178              

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[kvii]

@hawkingrei And could you help me to review this code too? This code removed unnecessary "log.Output" so it doubles the performance of stdLogger.