Reduce Minio access key minimum length to 3 (#5478)

This is a generic minimum value. The current reason is to support
Azure blob storage accounts name whose length is less than 5. 3 is the
minimum length for Azure.

cmd/jwt_test.go

@@ -39,8 +39,8 @@ func testAuthenticate(authType string, t *testing.T) {
 		secretKey   string
 		expectedErr error
 	}{
-		// Access key (less than 5 chrs) too small.
-		{"user", cred.SecretKey, auth.ErrInvalidAccessKeyLength},
+		// Access key (less than 3 chrs) too small.
+		{"u1", cred.SecretKey, auth.ErrInvalidAccessKeyLength},
 		// Secret key (less than 8 chrs) too small.
 		{cred.AccessKey, "pass", auth.ErrInvalidSecretKeyLength},
 		// Authentication error.