Use major version ref of Slack notification action

At the time the workflow was written the authors of the `rtCamp/action-slack-notify` GitHub Actions action did not
provide a major version ref. This meant that it was necessary to pin the action to a specific version.

Since then, a few new releases have been made, meaning an outdated version of the action was in use as a consequence of
the pinning.

The action now offers a `v2` major ref. Use of this ref will cause the workflow to benefit from ongoing development to
the action up until such time as a new major release is made, at which time we would need to evaluate whether any changes
to the workflow are required by the breaking change that triggered the major release before updating the major ref
(e.g., `uses: rtCamp/action-slack-notify@v3`).

Author: per1234

.github/workflows/check-certificates.yml

@@ -74,7 +74,7 @@ jobs:
             :warning::warning::warning::warning:
           SLACK_COLOR: danger
           MSG_MINIMAL: true
-        uses: rtCamp/action-slack-notify@v2.1.0
+        uses: rtCamp/action-slack-notify@v2
 
       - name: Get days remaining before certificate expiration date
         env:
@@ -128,4 +128,4 @@ jobs:
             :warning::warning::warning::warning:
           SLACK_COLOR: danger
           MSG_MINIMAL: true
-        uses: rtCamp/action-slack-notify@v2.1.0
+        uses: rtCamp/action-slack-notify@v2