Re-enable message posted on selective e2e tests detection

What does this MR do and why?

This is a follow-up of !126961 (merged) and mainly a revert of !123928 (merged) plus some message improvements.

Background

In !122896 (merged), we introduced a message/comment that would ask to start e2e:package-and-test-ee job on MR when selective e2e tests are detected. However, we came to know that in order to start e2e:package-and-test-ee job, the user would need to first start trigger-omnibus-env manually, then trigger-omnibus and then e2e:package-and-test-ee. This was bad UX. See this Slack thread (internal link) for the feedback and context. We decided to disable the message until we found a way to tie those jobs together so that they would run with once click which was done in !126961 (merged)

Changes in this MR

This MR brings back the message (temporarily removed in !123928 (merged)) that is posted when selective end-to-end tests are detected based on MR labels and also improves the message.

This MR also changes the reference of e2e:package-and-test to trigger-omnibus-and-follow-up-e2e in the Danger message that detects changes in qa selectors thus resolving #416304 (closed) and also improves the message.

Related to KR: https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/2295

Screenshots or screen recordings

Run trigger-omnibus-and-follow-up-e2e for selective tests

QA Selectors

Screenshot from: !127363 (comment 1485399015)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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

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