Update MAINTAINERS.md
[CI SKIP]
Update to MAINTAINERS.md base on newly adopted Governance and Maintainership definitions.
Maintainership is on a per project basis.
-### Core-maintainers
+### Maintainers
- Derek Collison <derek@nats.io> [@derekcollison](https://github.com/derekcollison)
- Ivan Kozlovic <ivan@nats.io> [@kozlovic](https://github.com/kozlovic)
- R.I. Pienaar <rip@devco.net> [@ripienaar](https://github.com/ripienaar)
-### Maintainers
-
