update webpush by minishlink to version 6

What does this MR do?

Upgrade webpush

How confident are you it won't break things if deployed?

It works nicely on localhost. But there is an error in the tests due to a bug of their package.

How to test

Enable Push Service on beta and see if it still works.

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text

Upgraded the minishlink webpush package to version 6.