use crypto/sha256 only for FIPS 140-2 compliance (#14983)

It would seem like the PR #11623 had chewed more
than it wanted to, non-fips build shouldn't really
be forced to use slower crypto/sha256 even for
presumed "non-performance" codepaths. In MinIO
there are really no "non-performance" codepaths.
This assumption seems to have had an adverse
effect in certain areas of CPU usage.

This PR ensures that we stick to sha256-simd
on all non-FIPS builds, our most common build
to ensure we get the best out of the CPU at
any given point in time.

internal/kms/single-key.go

@@ -22,7 +22,6 @@ import (
 	"crypto/aes"
 	"crypto/cipher"
 	"crypto/hmac"
-	"crypto/sha256"
 	"encoding/base64"
 	"errors"
 	"fmt"
@@ -33,6 +32,8 @@ import (
 	"github.com/secure-io/sio-go/sioutil"
 	"golang.org/x/crypto/chacha20"
 	"golang.org/x/crypto/chacha20poly1305"
+
+	"github.com/minio/minio/internal/hash/sha256"
 )
 
 // Parse parses s as single-key KMS. The given string