SecurityManager is deprecated for removal

`SecurityManager` is [deprecated for removal](https://openjdk.org/jeps/486) in JDK 24, but it's used in [`Native.getCallingClass()`](https://github.com/java-native-access/jna/blob/master/src/com/sun/jna/Native.java#L1525C1-L1540C1). That usage has to be removed in order for JNA to be compatible with newer versions of Java.