fix(push): use standalone registration endpoint

[marcin-kozinski]

Fix setting Background syncing to "Instantly (Push)", by switching from a (de)registration v3/send action to a stand-alone endpoint.

References

• https://mozilla-hub.atlassian.net/browse/POCKET-9883

PR Checklist

Setup:

• Described changes for automated release notes in PR title using
  Conventional Commits standard
• Self Review (review, clean up, documentation)
• Basic Self QA

Review:

• Code Review approved
• If modified GraphQL spec or queries, checked the usage file for invalid or no longer used definitions and cleaned it up if necessary