Use a dedicated project access token for leaving comments on upstream commits

This allows:

• The notify_upstream_commit:* jobs to use a new environment variable that contain dedicated Project access token instead of the default $GITLAB_BOT_MULTI_PROJECT_PIPELINE_POLLING_TOKEN variable which is a @gitlab-bot's personal access token that is abused a lot.

A similar change is made in the gitlab-org/gitlab project at gitlab!66548 (merged), and in the gitlab-org/omnibus-gitlab project at omnibus-gitlab!5487 (merged).

The following new project access tokens have been created:

• "Multi-pipeline (from 'gitlab-org/gitlab-qa' 'notify_upstream_commit:*' jobs)" at https://gitlab.com/gitlab-org/gitlab-qa-mirror/-/settings/access_tokens
  • Set as masked variable GITLAB_QA_PROJECT_ACCESS_TOKEN at https://gitlab.com/gitlab-org/gitlab-qa-mirror/-/settings/ci_cd

Related to https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/943.