Fix releasing notes

[jack-berg]

The RELEASING.md notes reference creating release branches which don't appear to exist: https://github.com/open-telemetry/opentelemetry-proto/branches

Can someone who has recently performed the release update the notes with the process?

Ran into this because we copied the releasing workflow for opentelemetry-configuration and ran into missing branch protection rules.

[arminru]

I think we'll also have to create a tag protection rule here: https://github.com/open-telemetry/opentelemetry-proto/settings/tag_protection

[arminru]

@tigrannajaryan can you take a look? Otherwise I can also update the docs and add a rule as described.

[tigrannajaryan]

We don't do release branches. We use the release function in github which adds tags.