chore: Update Slack invitation link

Updated the expired Firecracker slack workspace link
in the README with a new valid one.

Signed-off-by: Marco Cali <xmarcalx@amazon.co.uk>

Author: xmarcalx

README.md

@@ -171,7 +171,7 @@ You can get in touch with the Firecracker community in the following ways:
 
 - Security-related issues, see our [security policy document](SECURITY.md).
 - Chat with us on our
-  [Slack workspace](https://join.slack.com/t/firecracker-microvm/shared_invite/zt-1fecwrorm-x5URTlOzBR2fExTU2mWfug)
+  [Slack workspace](https://join.slack.com/t/firecracker-microvm/shared_invite/zt-1zlb87h4z-NED1rBhVqOQ1ygBgT76wlg)
   _Note: most of the maintainers are on a European time zone._
 - Open a GitHub issue in this repository.
 - Email the maintainers at