Replace iOS beta onboarding link with testflight

[drewvolz]

Swap out the defunct heroku/fastlane signup with testflight's link. This is a note so that we are aware that it's broken and that we should correct the link once we promote our next beta build.

[rye]

Triaging this… is there any place besides the README that we need to change?

[rye]

Oh, also—do we know who controls the boarding-aao Heroku app? Apparently it's throwing errors, and while we're changing to a public TestFlight link I think we should also deactivate that app.

[hawkrives]

Me. I'll try to remember to do that.

[rye]

@hawkrives I can't seem to find the login password for TF so you might have to take care of changing the link.

[rye]

I cannot generate the TestFlight link, however I have confirmed that I do have control over the boarding-aao Heroku app.

[drewvolz]

Closed by #6694