Switch from branch protection rules to rulesets

[trask]

Opening this for documentation purposes mainly.

Branch protection rules are being replace (for good reasons) by rulesets.

Unfortunately, there's an important feature (in OpenTelemetry repos at least) which is not available yet in rulesets: https://github.com/orgs/community/discussions/61107#discussioncomment-6569780.

I've experimented with moving to rulesets but keeping branch protection rules just for that one feature, and it kind of works, but then merge queues also need to be managed in the branch protection rule (instead of the ruleset), since otherwise GitHub will block the merge queue from merging to main. Basically, things get complicated and confusing, and I don't think it's worth it. Instead, I'd recommend we stay on classic branch protection rules for our core branch rules until the above feature is available 🤞.