Scheduled removal of Applets from Java

Hello,

Applet's terminal deprecation is finally being followed through on and is planned for removal in [Java 26](https://openjdk.org/projects/jdk/26/), probably in March 2026.

Over at AstroImageJ we're wondering how ImageJ plans to handle this. We don't make use of applets (I don't know if they are even possible to use anymore), and it seems feasible for us to make the changes without breaking the API, but thought it would be a good idea to give a headsup on the removal in case you missed it.

If you are interested in us making a PR for the removal, we could potentially do so though I am unsure of exactly how you'd want to handle the API surface that touches applets and if any of your users use them. My current thought is to make `IJ#getApplet` always return `null`, and anything that takes an `Applet` get replaced with an `Object` that throws when non null, though maybe `Void` would be better?

Start with the Java 26 removal notice and inspect the IJ#getApplet entry point plus the APIs that accept an Applet. The issue does not settle whether these APIs should be removed, return null, or use another replacement, so completion requires an API-handling decision before implementation.

Written by the indexing model from the issue text.