config: enforce AES-GCM in FIPS mode (#12265)

This commit enforces the usage of AES-256
for config and IAM data en/decryption in FIPS
mode.

Further, it improves the implementation of
`fips.Enabled` by making it a compile time
constant. Now, the compiler is able to evaluate
the any `if fips.Enabled { ... }` at compile time
and eliminate unused code.

Signed-off-by: Andreas Auernhammer <aead@mail.de>

pkg/fips/no_fips.go

@@ -15,6 +15,8 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+// +build !fips
+
 package fips
 
 import (
@@ -23,7 +25,7 @@ import (
 	"github.com/minio/sio"
 )
 
-var enabled = false
+const enabled = false
 
 func cipherSuitesDARE() []byte {
 	return []byte{sio.AES_256_GCM, sio.CHACHA20_POLY1305}