[v3] Update maintainers

@RoelofWobben, @macta, @bencoman, you are all listed as maintainers of the pharo-smalltalk track for v2. If you would like to still be [listed as maintainers](https://github.com/exercism/v3/blob/master/languages/pharo-smalltalk/maintainers.md) for v3 and continue contributing please let me know. Let me know if you want to be listed as a senior maintainer or a contributing maintainer. 