Slack invite is no longer valid

[JeftavanderHorst]

There are two invite links to the OpenTF Slack in the manifesto that are no longer valid:

https://github.com/opentffoundation/manifesto/blob/ba197da1cff8df3ff519428dd8b31d904eeb8ddf/fork.html#L91
https://github.com/opentffoundation/manifesto/blob/ba197da1cff8df3ff519428dd8b31d904eeb8ddf/fork.html#L125

[abhayporwals]

Hey @JeftavanderHorst, Thanks for opening the issue !
We will try to resolve this as soon as possible.