unpin slack_sdk version
# What does this PR do?

The issue in Slack SDK 3.18.2 was fixed in 3.18.3, so no longer need to pin 3.18.1.

For more details, see
https://github.com/slackapi/python-slack-sdk/pull/1259#issuecomment-1237731450
https://github.com/slackapi/python-slack-sdk/issues/1261
_The documentation is not available anymore as the PR was closed or merged._