Update CODEOWNERS (#106)

* Update CODEOWNERS

* remove old maintainers from readme

Author: mickmister

CODEOWNERS

@@ -1 +1 @@
-* @catalintomai
+* @mickmister

README.md

@@ -5,8 +5,7 @@
 [![Release](https://img.shields.io/github/v/release/mattermost/mattermost-plugin-welcomebot)](https://github.com/mattermost/mattermost-plugin-welcomebot/releases/latest)
 [![HW](https://img.shields.io/github/issues/mattermost/mattermost-plugin-welcomebot/Up%20For%20Grabs?color=dark%20green&label=Help%20Wanted)](https://github.com/mattermost/mattermost-plugin-welcomebot/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3A%22Up+For+Grabs%22+label%3A%22Help+Wanted%22)
 
-**Maintainer:** [@jfrerich](https://github.com/jfrerich)
-**Co-Maintainer:** [@iomodo](https://github.com/iomodo)
+**Maintainer:** [@mickmister](https://github.com/mickmister)
 
 Use this plugin to improve onboarding and HR processes. It adds a Welcome Bot that helps welcome users to teams and/or channels as well as easily join channels based on selections.