Fix release workflow issues

[nirinchev]

This adds an empty changelog section as well as tweaks a few things about the release workflow

1. Disable fail-fast for the installation test job as we want to try and test all installation methods, even if some of them fail.
2. Uses https://github.com/marketplace/actions/retry-step to retry installation tests on failure. Currently set to 10 retries with 60 seconds between them. This is needed due to CocoaPods not indexing a new release quickly enough.
3. Parallelize some of the steps - i.e. the post to slack/update changelog jobs don't need to be waiting for the installation tests.
4. Bump the release-to-slack action to latest main. It includes a bugfix where not being able to transform markdown to slack blocks won't fail the job, but rather will post a message with a link to the release instead.