Slack invite link is broken in README.md

[vladibb]

Description:

While carefully and optimistically navigating into the GitHub landing page of the SAM project, scrolling through the text and finding URL "Join the SAM developers channel (#samdev)" and clicking on it, leads to an expired Slack URL.

Have the community moved somewhere else? which slack? or everybody migrating to Teams?

Steps to reproduce the issue:

1. Navigate to https://github.com/awslabs/serverless-application-model
2. Press on "Join the SAM developers channel (#samdev)"

Observed result:
This invite link is no longer active. Please check with the person who shared it with you to see if there's a new link available.

Expected result:
🍺

[rubyrose-alexis]

Observed the same behavior today. When can an active link be placed?