feat(doc): update slack badge

Signed-off-by: ngjaying <ngjaying@gmail.com>

README-CN.md

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 # EMQ X Kuiper - 超轻量物联网边缘数据分析软件
 [![GitHub Release](https://img.shields.io/github/release/emqx/kuiper?color=brightgreen)](https://github.com/emqx/kuiper/releases)
 [![Docker Pulls](https://img.shields.io/docker/pulls/emqx/kuiper)](https://hub.docker.com/r/emqx/kuiper)
-[![Slack Invite](<https://kuiper-slack-invite.emqx.io/badge.svg>)](https://kuiper-slack-invite.emqx.io/)
+[![Slack](https://img.shields.io/badge/Slack-LF%20Edge-39AE85?logo=slack)](https://join.slack.com/t/lfedge/shared_invite/zt-7kavdtmq-SeyFzM2CEABBcKYGEVCgkw)
 [![Twitter](https://img.shields.io/badge/Follow-EMQ-1DA1F2?logo=twitter)](https://twitter.com/EMQTech)
 [![问答社区](https://img.shields.io/badge/问答社区-Kuiper-yellow)](https://askemq.com/c/kuiper/)
 [![YouTube](https://img.shields.io/badge/Subscribe-EMQ%20中文-FF0000?logo=youtube)](https://www.youtube.com/channel/UCir_r04HIsLjf2qqyZ4A8Cg)

README.md

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 [![GitHub Release](https://img.shields.io/github/release/emqx/kuiper?color=brightgreen)](https://github.com/emqx/kuiper/releases)
 [![Docker Pulls](https://img.shields.io/docker/pulls/emqx/kuiper)](https://hub.docker.com/r/emqx/kuiper)
-[![Slack Invite](<https://kuiper-slack-invite.emqx.io/badge.svg>)](https://kuiper-slack-invite.emqx.io/)
+[![Slack](https://img.shields.io/badge/Slack-LF%20Edge-39AE85?logo=slack)](https://join.slack.com/t/lfedge/shared_invite/zt-7kavdtmq-SeyFzM2CEABBcKYGEVCgkw)
 [![Twitter](https://img.shields.io/badge/Follow-EMQ-1DA1F2?logo=twitter)](https://twitter.com/EMQTech)
 [![Community](https://img.shields.io/badge/Community-Kuiper-yellow?logo=github)](https://github.com/emqx/kuiper/discussions)
 [![YouTube](https://img.shields.io/badge/Subscribe-EMQ-FF0000?logo=youtube)](https://www.youtube.com/channel/UC5FjR77ErAxvZENEWzQaO5Q)