Release Note: JEP 407: Remove RMI Activation

The Remote Method Invocation (RMI) Activation mechanism has been removed. RMI Activation was an obsolete part of RMI that has been optional since Java SE 8. RMI Activation was deprecated for removal by [JEP 385](https://openjdk.java.net/jeps/385) in Java SE 15, and it was removed from this release by [JEP 407](https://openjdk.java.net/jeps/407). The `rmid` tool has also been removed. See JEP 385 for background, rationale, risks, and alternatives. The rest of RMI remains unchanged.