Update Slack invitation link on Website and README

[wuchong]

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Description

Currently, we are still using the legacy fluss-hq workspace link, and should update them with the new inviation link:

https://join.slack.com/t/apache-fluss/shared_invite/zt-33wlna581-QAooAiCmnYboJS8D_JUcYw:

Willingness to contribute

• I'm willing to submit a PR!

[wuchong]

cc @MehulBatra , do you want to take this?