Use crypto/sha256 (#29386)

Go 1.21 improved the performance of `crypto/sha256`. It's now similar to
`minio/sha256-simd`, so we should just use the standard libs.

https://go.dev/doc/go1.21#crypto/sha256
https://go-review.googlesource.com/c/go/+/408795
https://github.com/multiformats/go-multihash/pull/173

modules/auth/password/hash/pbkdf2.go

@@ -4,12 +4,12 @@
 package hash
 
 import (
+	"crypto/sha256"
 	"encoding/hex"
 	"strings"
 
 	"code.gitea.io/gitea/modules/log"
 
-	"github.com/minio/sha256-simd"
 	"golang.org/x/crypto/pbkdf2"
 )