Update changes.md for 2.4.0 release

[pnoltes]

The CHANGES.md file contains a list of noteworthy changes up to Apache Celix 2.3.0, this should be updated up to Apache Celix 2.4.0 (next release).

[PengZheng]

Yes. We already have lots of goodies for a new release.
The experimental Rust POC is an exciting addition. Since it relies on the current shell, which I plan to redesign (#499) in 3.0.0, let's get it merged and include it in 2.4.0.

Oh, I have to mention that before 2.4.0 happens, I also wanna have some small improvements of shell merged (#629).