docs(coc): bump from 1.4 to 2.0

[Fdawgs]

Brings Code of Conduct into line with the OpenJS Foundation's Code of Conduct.

Already updated in main repo: https://github.com/fastify/fastify/pull/2857 😸

Checklist

• [x] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct