build: promote on main branch, not master. Remove stale Labels workflow

[jbw976]

Description of your changes

After merging #32, it looks like we are getting successful CI runs from main: https://github.com/crossplane/conformance/actions/runs/17601691824 🎉

However, there are a couple problems that this PR aims to address:

• promote is being skipped on builds in main because we're checking for refs/heads/master and the branch has been renamed to main.
• the Labels workflow is constantly failing because it has no config file, but I don't think this workflow is needed at all - it has been removed

I've also tagged main with v2.0.0-cf.1-rc.0 so we start getting nice version numbers indicating we're working towards (and getting close) to the v2.0.0-cf.1 release.

I have:

• Read and followed conformance's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

CI will test all of this 🙏