Hide utility class constructors

Update all utility classes so that they have a private constructor. This
prevents users from accidentally creating an instance, when they should
just use the static methods directly.

Issue gh-8945

crypto/src/test/java/org/springframework/security/crypto/codec/Base64Tests.java

@@ -28,8 +28,6 @@ public class Base64Tests {
 
 	@Test
 	public void isBase64ReturnsTrueForValidBase64() {
-		new Base64(); // unused
-
 		assertThat(Base64.isBase64(new byte[] { (byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D' })).isTrue();
 	}
 

crypto/src/test/java/org/springframework/security/crypto/encrypt/CryptoAssumptions.java

@@ -26,7 +26,10 @@ import org.junit.AssumptionViolatedException;
 
 import org.springframework.security.crypto.encrypt.AesBytesEncryptor.CipherAlgorithm;
 
-public class CryptoAssumptions {
+public final class CryptoAssumptions {
+
+	private CryptoAssumptions() {
+	}
 
 	public static void assumeGCMJCE() {
 		assumeAes256(CipherAlgorithm.GCM);