chore(repo): update old heroku app links (#27284)

Issue number: N/A

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## What is the current behavior?
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there are a few outdated links to a dead heroku app in the repo

## What is the new behavior?

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this PR. -->
this commit removes old links within the repo that point to a heroku app
that is no longer working/maintained. the original intent was to update
stencil-related links initially (as stencil still focuses on slack for
community communications). however, there were two references to the
ionic worldwide slack, which has been since replaced with discord. i've
updated those link (which were only in comments) as well. after applying
this commit, there are no references to 'heroku' in the repo

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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Feel free to reject this/request we remove these links altogether. I'm
not entirely sure we want to continue to provide them in the `core/` and
`angular/` directories (when we don't have them in `packages/react/` nor
`packages/vue/`