Position essential information at the top of notification emails

### Problem Statement

Since updating our self-hosted Sentry instance to 22.4.0, notification emails contain a button titled `Set up in Slack` as well as other non essential hints at the top.


We are not using the Slack integration, and we do not plan to use it. Previously in some 20.x version, the email was more concise and showed important information more prominently.


### Solution Brainstorm

Notification emails are not the right place to inform users about possible ways to use Sentry, their primary purpose should be informing about the issue at hand. Non-essential information should either be omitted or further down after the primary content.