Use sentence case for trigger events on the integrations form

What does this MR do and why?

Use sentence case for trigger events on the integrations form. They are passed as externalized strings from the backend instead of using string transforms to display them on the frontend.

Changelog: changed

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to Project > Settings > Integrations, choose an integration that uses triggers like Slack. For example, http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/integrations/slack/edit.

Observe that the trigger labels next to each checkbox is now sentence case.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #328682 (closed)