update slack inviter link

[thesuperzapper]

This PR updates our Kubeflow Slack inviter link to use the same system as CNCF Slack, the current bot is struggling under the large number of people joining for GSoC.

See these related user issues:

• [Slack] I didn't received the email after I submit to join kubeflow slack community#706
• https://groups.google.com/g/kubeflow-discuss/c/ncc_piQY2O8

The link is changing from:

• OLD: https://invite.playplay.io/invite?team_id=T7QLHSH6U
• NEW: https://communityinviter.com/apps/kubeflow/slack

---

NOTE: The new link still invites users to the Kubeflow slack, not the CNCF one. We are still planning on how to migrate to CNCF slack so we can minimize disruption.

[thesuperzapper]

@andreyvelich once we get confirmation from the people who can't join using the old link, let's merge this.

(Assuming the new link works for them).

[andreyvelich]

Thanks for the update @thesuperzapper!
/lgtm
/approve

[google-oss-prow]

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