Add Matt Robenolt to MAINTAINERS.md (#593)

This PR proposes adding Matt Robenolt (@mattrobenolt), from PlanetScale,
to the list of maintainers. Per [Connect's governance
policy](https://connectrpc.com/docs/governance/project-governance), this
also makes Matt one of the handful of maintainers with a vote on Connect
RFCs.

Matt was one of Connect's earliest enthusiasts. He provided feedback on
Connect's early releases, routinely helps other users in issues, and
often reviews pull requests. He and his team also maintain a
high-traffic Connect deployment, so his operational feedback has been
incredibly valuable. Thanks for your contributions, Matt!

MAINTAINERS.md

@@ -5,6 +5,7 @@ Maintainers
 * [Peter Edge](https://github.com/bufdev), [Buf](https://buf.build)
 * [Akshay Shah](https://github.com/akshayjshah), [Buf](https://buf.build)
 * [Josh Humphries](https://github.com/jhump), [Buf](https://buf.build)
+* [Matt Robenolt](https://github.com/mattrobenolt), [PlanetScale](https://planetscale.com)
 
 ## Former
 * [Alex McKinney](https://github.com/amckinney)