MM-51683 Re-add CODEOWNERS for Web Platform files (#22627)

webapp/channels/CODEOWNERS

@@ -1,4 +0,0 @@
-# Web Platform should be assigned to review all PRs that involve changing dependencies in the app, either intentional or accidental.
-package.json @mattermost/web-platform
-*/package.json @mattermost/web-platform
-package-lock.json @mattermost/web-platform