Deprecate Toolkit CODEOWNERS (#22907)

* Deprecate Toolkit CODEOWNERS

* change code owner to `@mattermost/integrations`

* remove code owner

---------

Co-authored-by: Michael Kochell <6913320+mickmister@users.noreply.github.com>

CODEOWNERS

@@ -1,9 +1,7 @@
-/plugin/ @mattermost/toolkit
-
 /.github/workflows/channels-ci.yml @mattermost/web-platform
 /webapp/package.json @mattermost/web-platform
 /webapp/channels/package.json @mattermost/web-platform
 /webapp/Makefile @mattermost/web-platform
 /webapp/package-lock.json @mattermost/web-platform
 /webapp/platform/*/package.json @mattermost/web-platform
-/webapp/scripts @mattermost/web-platform
+/webapp/scripts @mattermost/web-platform