fix: replace CircleCI env vars in post-release Slack notifications

Also removes duplicate root-level scripts (post-release.sh,
release-wporg.sh) in favor of scripts/github/ versions, and updates
the legacy CircleCI job reference in src/jobs/post-release.yml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: adekbadek

post-release.sh

@@ -35,7 +35,7 @@ else
     else
       echo '[newspack-scripts] Notifying the team on Slack.'
       curl \
-        --data "{\"channel\":\"$SLACK_CHANNEL_ID\",\"blocks\":[{\"type\":\"section\",\"text\":{\"type\":\"mrkdwn\",\"text\":\"⚠️ Post-release merge to alpha failed for: \`$CIRCLE_PROJECT_REPONAME\`. Check <$CIRCLE_BUILD_URL|the build> for details.\"}}]}" \
+        --data "{\"channel\":\"$SLACK_CHANNEL_ID\",\"blocks\":[{\"type\":\"section\",\"text\":{\"type\":\"mrkdwn\",\"text\":\"⚠️ Post-release merge to alpha failed for: \`$GITHUB_REPOSITORY\`. Check <$GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID|the build> for details.\"}}]}" \
         -H "Content-type: application/json" \
         -H "Authorization: Bearer $SLACK_AUTH_TOKEN" \
         -X POST https://slack.com/api/chat.postMessage \
@@ -61,7 +61,7 @@ else
   else
     echo '[newspack-scripts] Notifying the team on Slack.'
     curl \
-      --data "{\"channel\":\"$SLACK_CHANNEL_ID\",\"blocks\":[{\"type\":\"section\",\"text\":{\"type\":\"mrkdwn\",\"text\":\"⚠️ Post-release merge to \`trunk\` failed for: \`$CIRCLE_PROJECT_REPONAME\`. Check <$CIRCLE_BUILD_URL|the build> for details.\"}}]}" \
+      --data "{\"channel\":\"$SLACK_CHANNEL_ID\",\"blocks\":[{\"type\":\"section\",\"text\":{\"type\":\"mrkdwn\",\"text\":\"⚠️ Post-release merge to \`trunk\` failed for: \`$GITHUB_REPOSITORY\`. Check <$GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID|the build> for details.\"}}]}" \
       -H "Content-type: application/json" \
       -H "Authorization: Bearer $SLACK_AUTH_TOKEN" \
       -X POST https://slack.com/api/chat.postMessage \

release-wporg.sh

@@ -7,4 +7,4 @@ steps:
   - checkout_with_workspace
   - run:
       name: Perform post-release chores
-      command: ./node_modules/newspack-scripts/post-release.sh
+      command: ./node_modules/newspack-scripts/scripts/github/post-release.sh