Separate mentions and assignees in stage/group reports

Context

• #1094 (closed) was complaining that an assignee was mentioned twice in triage reports.
• We implemented !1772 (merged) (by yours truly 🙈) to fix the problem, but this MR inadvertently disabled mentions for the people/groups listed in the extra_mentions: array.

What does this MR do and why?

• Remove the assignees from the mentions array.
• Make the people present in mentions: mentioned again in reports.
• Ensure that we don't ping an assignees in the issue description (they will receive a ping when being assigned).
• Add specs for the main reports present in https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/lib/group_triage_helper.rb.

Expected impact & dry-runs

Most triage report will be impacted. People present in the extra_mentions array should be notified in the next triage issue.

I picked one report as an example:

• Group: Pipeline Authoring (because they have extra_assignees: and extra_mentions: defined)
• Policy: policies/generated/team-summary.yml.erb/pipeline_authoring.yml
• Dry-run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/4806895202#L315:

* Title: 2023-08-04 - Triage report for "group::pipeline authoring" - gitlab-org/gitlab
* Description: Hi `@dhershkovitch` `@marknuzzo` `@treagitlab` `@rayana` @sunjungp,

Looking at https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/317c08ce0125f4c3f216994be6f4ea9384782a72/group-definition.yml#L156-167, we only want the people mentioned in extra_mentions: to be pinged, so 🎉

(The assignees as also correctly set)

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review