docs: Update Discord invitation link (backport #4366)

[mergify]

Update Discord invitation link in the Readme

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This is an automatic backport of pull request #4366 done by Mergify.

[mergify]

Cherry-pick of 2909bd2 has failed:

On branch mergify/bp/release/v28.x.y/pr-4366
Your branch is up to date with 'origin/release/v28.x.y'.

You are currently cherry-picking commit 2909bd24.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   readme.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[julienrbrt]

@toschdev could you fix the conflicts here?

[julienrbrt]

This link is broken again