Release version 0.6.0

Git ref: f2df79dc

Checklist

Basics

1. The CI pipeline succeeded, including all unit tests and code quality checks. (https://git.ligo.org/emfollow/gwcelery/pipelines/62874)
2. CHANGES.rst lists all significant changes since the last release. It is free from spelling and grammatical errors.

Playground deployment

3. The playground deployment run for at least 10 minutes.
4. The Flower monitor is reachable and shows no unexpected task failures.
5. The Flask dashboard is reachable.
6. The playground deployment is connected to LVAlert.
7. The playground deployment is connected to GCN (receiving only).

Mock events

8. The playground deployment has produced an MDC superevent. https://gracedb-playground.ligo.org/superevents/MS190520z/view/
9. The MDC superevent has the following annotations.
   • bayestar.fits
   • bayestar.fits.gz
   • bayestar.png
   • bayestar.volume.png
   • bayestar.html
   • p_astro.json
   • p_astro.png
   • em_bright.json
10. The MDC superevent has the following labels.
    • EMBRIGHT_READY
    • GCN_PRELIM_SENT
    • PASTRO_READY
    • SKYMAP_READY
11. The MDC superevent has a preliminary VOEvent.
12. The MDC superevent has either an ADVOK or an ADVNO label.
13. Issuing an ADVOK signoff through GraceDB results in an initial VOEvent.
14. Issuing an ADVNO signoff through GraceDB results in a retraction VOEvent.
15. Requesting an update alert through the Flask dashboard results in an update VOEvent.