Update Maintainers (#283)

I propose adding @jhump as a maintainer to replace @akshayjshah. Akshay
and I discussed this as well, and given Josh's work on Connect-Kotlin
and the conformance suite, I think he has sufficient context to help
maintain this project.

MAINTAINERS.md

@@ -3,6 +3,6 @@ Maintainers
 
 ## Current
 * [Peter Edge](https://github.com/bufdev), [Buf](https://buf.build)
-* [Akshay Shah](https://github.com/akshayjshah), [Buf](https://buf.build)
 * [Michael Rebello](https://github.com/rebello95), [Airbnb](https://airbnb.com)
+* [Josh Humphries](https://github.com/jhump), [Buf](https://buf.build)
 * [Eddie Seay](https://github.com/eseay), [Chick-fil-A](https://www.chick-fil-a.com/)