Update MAINTAINERS.md [ci skip]
[CI SKIP]
Update to MAINTAINERS.md base on newly adopted Governance and Maintainership definitions.
Maintainership is on a per project basis.
-### Core-maintainers
+### Maintainers
- Alberto Ricart <alberto@nats.io> [@aricart](https://github.com/aricart)
- Stephen Asbury <sasbury@sasbury.com> [@sasbury](https://github.com/sasbury)
- Colin Sullivan <colin@nats.io> [@ColinSullivan1](https://github.com/ColinSullivan1)
