[MM-61079] Fix error checking in webhook.go (#30611)

* MM-28751 Fix error checking in webhook.go

- Implement proper error checking for r.ParseForm()
- Implement proper error checking for r.ParseMultipartForm()
- Implement proper error checking for w.Write()
- Remove webhook.go exception from .golangci.yml

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Co-Authored-By: Claude <noreply@anthropic.com>

* Fix translation

---------

Co-authored-by: Claude <noreply@anthropic.com>

server/.golangci.yml

@@ -165,7 +165,6 @@ issues:
         channels/web/oauth_test.go|\
         channels/web/saml.go|\
         channels/web/web_test.go|\
-        channels/web/webhook.go|\
         cmd/mattermost/commands/cmdtestlib.go|\
         cmd/mattermost/commands/db.go|\
         cmd/mattermost/commands/export.go|\