[CI] Flag PR branches named main

[chalin]

Originally posted by @chalin in #5607 (review):

As documented in https://opentelemetry.io/docs/contributing/pull-requests/#pr-guidelines:

• Do not work from the main branch of your fork, but create a PR-specific
  branch.

I'm not sure that you can rename the branch bound to a Github PR, or if it is worth it. I'll let @theletterf and you decide @Victorsesan.

[chalin]

From #6388:

Our contribution guidelines ask that PRs be submitted with a branch name other than main. I'd like to suggest that we add a simple GH PR check to report this when the PR is first submitted. Otherwise, we maintainers often only find out when it's "too late", i.e., we've gone through rounds of reviews, and want to submit a fix (either via our /fix commands or manually), but can't. WDYT @open-telemetry/docs-approvers?

[chalin]

PR in support of this:

• Automated GitHub Action to Prevent PRs from Main Branch #6440