Rename SlackNotificationService back to SlackService (!8208) · Merge requests · GitLab.org / GitLab FOSS · GitLab

What does this MR do?

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8191#note_20310845

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25843

/cc @yorickpeterse @stanhu @smcgivern @felipe_artur @ayufan