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

config/src/test/java/org/springframework/security/config/annotation/web/configurers/saml2/TestSaml2Credentials.java

@@ -29,7 +29,10 @@ import org.springframework.security.saml2.credentials.Saml2X509Credential.Saml2X
 /**
  * Preconfigured SAML credentials for SAML integration tests.
  */
-public class TestSaml2Credentials {
+public final class TestSaml2Credentials {
+
+	private TestSaml2Credentials() {
+	}
 
 	static Saml2X509Credential verificationCertificate() {
 		String certificate = "-----BEGIN CERTIFICATE-----\n"

config/src/test/java/org/springframework/security/config/annotation/web/reactive/ServerHttpSecurityConfigurationBuilder.java

@@ -26,7 +26,10 @@ import org.springframework.security.core.userdetails.ReactiveUserDetailsService;
  * @author Rob Winch
  * @since 5.0
  */
-public class ServerHttpSecurityConfigurationBuilder {
+public final class ServerHttpSecurityConfigurationBuilder {
+
+	private ServerHttpSecurityConfigurationBuilder() {
+	}
 
 	public static ServerHttpSecurity http() {
 		return new ServerHttpSecurityConfiguration().httpSecurity();

config/src/test/java/org/springframework/security/config/http/SecurityFiltersAssertions.java

@@ -29,13 +29,15 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Josh Cummings
  */
-public class SecurityFiltersAssertions {
+public final class SecurityFiltersAssertions {
 
 	private static Collection<SecurityFilters> ordered = Arrays.asList(SecurityFilters.values());
 
+	private SecurityFiltersAssertions() {
+	}
+
 	public static void assertEquals(List<String> filters) {
 		List<String> expected = ordered.stream().map(SecurityFilters::name).collect(Collectors.toList());
-
 		assertThat(filters).isEqualTo(expected);
 	}