In Java 23, Subject.getSubject requires setting the system property java.security.manager to allow, else it will throw an exception.  More detail is available in the release notes: https://jdk.java.net/23/release-notes

This is in support of the eventual removal of the security manager, at which point, Subject.getSubject will be removed.  Since this project supports older Java releases, and the API which one must migrate to was only introduced in Java 18, I hid the implementations behind a runtime version check.  I verified against several local Java versions that the correct classes are loaded (and more importantly, that the incorrect classes are _not_ loaded).  The outright removal of the security manager, and by extension the Subject.getSubject method would be a particularly large problem if we do not address this, which was my motivation here.

Some manual testing on Java 11, 17, and 23:

```
» java --version
openjdk 11.0.23 2024-04-16 LTS
» java -cp hadoop-auth-3.5.0-SNAPSHOT.jar org.apache.hadoop.util.subject.SubjectAdapter
Current subject is null

» java --version
openjdk 17.0.5 2022-10-18 LTS
» java -cp hadoop-auth-3.5.0-SNAPSHOT.jar org.apache.hadoop.util.subject.SubjectAdapter
Current subject is null

» java --version
openjdk 23 2024-09-17
» java -cp hadoop-auth-3.5.0-SNAPSHOT.jar org.apache.hadoop.util.subject.SubjectAdapter
Current subject is null
```

As desired, the class containing the old implementation does not get loaded on a new JVM:
```
» java --version
openjdk 23 2024-09-17
» java -verbose:class -cp hadoop-auth-3.5.0-SNAPSHOT.jar org.apache.hadoop.util.subject.SubjectAdapter | \grep -i subject | grep "class,load" | awk '{print $2}'
org.apache.hadoop.util.subject.SubjectAdapter
org.apache.hadoop.util.subject.HiddenGetSubject
javax.security.auth.Subject
org.apache.hadoop.util.subject.GetSubjectNg
```

And the inverse is true for an older JVM:
```
» java --version
openjdk 17.0.5 2022-10-18 LTS
» java -verbose:class -cp hadoop-auth-3.5.0-SNAPSHOT.jar org.apache.hadoop.util.subject.SubjectAdapter | \grep -i subject | grep "class,load" | awk '{print $2}'
org.apache.hadoop.util.subject.SubjectAdapter
org.apache.hadoop.util.subject.HiddenGetSubject
javax.security.auth.Subject
org.apache.hadoop.util.subject.ClassicGetSubject
javax.security.auth.Subject$1
```