Update transparency-dev slack invite link

Author: taknira

README.md

@@ -16,7 +16,7 @@ repository requires Go version 1.23.
 
 ## Support
 
-- Slack: https://transparency-dev.slack.com/ ([invitation](https://join.slack.com/t/transparency-dev/shared_invite/zt-27pkqo21d-okUFhur7YZ0rFoJVIOPznQ))
+- Slack: https://transparency-dev.slack.com/ ([invitation](https://transparency.dev/slack/))
 
 ## Repository Structure