Slack Notifications: Update messaging to say removal planned for 18.0

This issue came out of this discussion: #424190 (comment 1691063650).

About

The Slack Notifications integration is deprecated. We have been announcing that the integration will be removed in 17.0, however there are a few blocking issues to its removal (mentioned in &8673) that have not been done (as of time of writing, in 16.8).

We will extend the milestone we plan to remove the Slack Notifications integration from 17.0 to 18.0 in order to give us time to complete those features.

assigned to @.luke

added to epic &8673

mentioned in issue #424190

changed milestone to %16.8

added Category:Integrations Deliverable IntegrationSlack groupimport and integrate maintenancerefactor typemaintenance workflowin dev labels

added devopsmanage sectioncore platform labels

set weight to 1

mentioned in epic &8673

mentioned in epic &1211 (closed)

changed the description

added workflowin review label and removed workflowin dev label

mentioned in commit c63a5868

changed the description

mentioned in merge request !140272 (merged)

mentioned in commit 11c93c74

Setting health status to on track as the milestone has just begun.

Issue participants are welcome to override this by setting the health status to another value.

changed health status to on track

mentioned in commit 73187d9f

mentioned in commit 79048fbd

Closing is this has been done in !140272 (merged).

closed

added workflowcomplete label and removed workflowin review label

mentioned in issue #391526 (closed)

mentioned in epic gitlab-org#8673