fix(notifications): accept all 2xx statuses in discord webhook sender

The discord sender was the last notification webhook sender that rejected
HTTP 202 Accepted (it only accepted 200/204), producing
'could not send notification via notifier: got status 202' for any
endpoint that acknowledges asynchronously. Accept the full 2xx range
(200 <= status < 300) like the generic webhook, mattermost and teams
senders already do, keeping a clear 'expected 2xx' error for non-2xx.

Extends TestDiscordWebhookStatusHandling to assert 200, 201, 202 and
204 succeed and 3xx/4xx/5xx return the status error.

Fixes #827

Spec-Ref: helix-specs@9926d75c:000399_fix-notification-webhook