fix(slack): only pass tags in metadata, not callback_id

[cathteng]

Fixes SENTRY-3DEY

The callback_id was used as both the callback_id and private_metadata. If we pass tags in the callback_id, sometimes it gets too long (has max length of 255 chars). Separate out creating the callback_id and private_metadata strings and only put the tags in the metadata will fix this.

https://github.com/getsentry/sentry/blob/22e0b9819a9af4518e3c55d74b6bb87854e314da/src/sentry/integrations/slack/webhooks/action.py#L351-L352

(^ getting rid of this!)