Switch to trusted publishing workflow and run it on version.rb changes

[zendesk-ops-ci]

Switches to a Trusted Publishing workflow.
This workflow will run whenever version.rb is changed in the main branch.
Setting ENV["gem_push"] is also not necessary anymore.
This workflow relies on the main branch being named main. Please rename it (main is the standard name across Zendesk).
bundler-cache for setup-ruby is set to false to prevent cache poisoning attacks.
Finally, we add a description of the new release process. Our README and CONTRIBUTING files don’t have a standard structure, so please check that we have updated a correct file and that the new section makes sense for this gem.

• Release description has been updated correctly.
• The main branch is renamed from master to main.