A few updates to JEP 411: Deprecate the Security Manager for Removal

[Sean Mullan]

JEP 411 has been updated with a few changes:

1. The "Description" section [1] has been updated with more details on 
our plans for phasing out the Security Manager in Java 18 and later.

2. The "Issue warnings" section [2] has been updated with the warning 
messages that are emitted at startup and run time when a Security 
Manager has been enabled.

3. JEP 411 has been moved to the Completed state as all required tasks 
have been completed.

Thanks,
Sean

[1] https://openjdk.java.net/jeps/411#Description
[2] https://openjdk.java.net/jeps/411#Issue-warnings