Fix equals nullability annotations for jspecify compliance

In this commit, we added `@Nullable` to equals methods of classes that
support `jspecify` for consistency with other Spring projects and to
avoid bugs that caused other Spring projects to do this natively.

Closes: gh-18929, gh-18927

Signed-off-by: Andrey Litvitski <andrey1010102008@gmail.com>

kerberos/kerberos-core/src/main/java/org/springframework/security/kerberos/authentication/KerberosServiceRequestToken.java

@@ -99,13 +99,16 @@ public class KerberosServiceRequestToken extends AbstractAuthenticationToken imp
 	 * equals() is based only on the Kerberos token
 	 */
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		if (this == obj) {
 			return true;
 		}
 		if (!super.equals(obj)) {
 			return false;
 		}
+		if (obj == null) {
+			return false;
+		}
 		if (getClass() != obj.getClass()) {
 			return false;
 		}