.github/workflows: address slack API method style breaking changes

These workflows were incorrectly updated during the first round of attempted changes, as they are actually the "Slack API Method" technique, not "Slack incoming webhook". The changes include:

- explicitly setting the `method` to `chat.PostMessage`
- passing the token in the `token` argument, rather than the `SLACK_BOT_TOKEN` environment variable
- sending the channel directly in the `payload`, rather than using the `channel-id` argument

.github/workflows/label-notifications.yml

@@ -24,13 +24,12 @@ jobs:
         uses: slackapi/slack-github-action@485a9d42d3a73031f12ec201c457e2162c45d02d # v2.0.0
         env:
           ISSUE_TITLE: ${{ toJSON(github.event.issue.title || github.event.pull_request.title) }}
-          SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
         with:
-          channel-id: ${{ secrets.SLACK_CHANNEL }}
-          webhook: ${{ secrets.SLACK_WEBHOOK_URL }}
-          webhook-type: incoming-webhook
+          method: chat.postMessage
+          token: ${{ secrets.SLACK_BOT_TOKEN }}
           payload: |
             {
+              "channel" : "${{ secrets.SLACK_CHANNEL }}",
               "blocks": [
                 {
                   "type": "section",

.github/workflows/post_publish.yml

@@ -107,14 +107,12 @@ jobs:
       - name: Send Slack Notification Upon Failure
         if: ${{ failure() }}
         uses: slackapi/slack-github-action@485a9d42d3a73031f12ec201c457e2162c45d02d # v2.0.0
-        env:
-          SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
         with:
-          channel-id: ${{ secrets.SLACK_CHANNEL }}
-          webhook: ${{ secrets.SLACK_WEBHOOK_URL }}
-          webhook-type: incoming-webhook
+          method: chat.postMessage
+          token: ${{ secrets.SLACK_BOT_TOKEN }}
           payload: |
             {
+              "channel" : "${{ secrets.SLACK_CHANNEL }}",
               "blocks": [
                 {
                   "type": "section",