use crypto/sha256 for FIPS 140-2 compliance (#11623)

This commit replaces the usage of
github.com/minio/sha256-simd with crypto/sha256
of the standard library in all non-performance
critical paths.

This is necessary for FIPS 140-2 compliance which
requires that all crypto. primitives are implemented
by a FIPS-validated module.

Go can use the Google FIPS module. The boringcrypto
branch of the Go standard library uses the BoringSSL
FIPS module to implement crypto. primitives like AES
or SHA256.

We only keep github.com/minio/sha256-simd when computing
the content-SHA256 of an object. Therefore, this commit
relies on a build tag `fips`.

When MinIO is compiled without the `fips` flag it will
use github.com/minio/sha256-simd. When MinIO is compiled
with the fips flag (go build --tags "fips") then MinIO
uses crypto/sha256 to compute the content-SHA256.

cmd/streaming-signature-v4.go

@@ -21,6 +21,7 @@ package cmd
 import (
 	"bufio"
 	"bytes"
+	"crypto/sha256"
 	"encoding/hex"
 	"errors"
 	"hash"
@@ -31,7 +32,6 @@ import (
 	humanize "github.com/dustin/go-humanize"
 	xhttp "github.com/minio/minio/cmd/http"
 	"github.com/minio/minio/pkg/auth"
-	sha256 "github.com/minio/sha256-simd"
 )
 
 // Streaming AWS Signature Version '4' constants.