contributing: add steps for adding and removing rc branches (#5223)

## Description

When doing a release the former rc branch should be deleted as the content of the branch is now on the release branch. After a release is made a new RC branch should be created. 

Closes: #XXX

CONTRIBUTING.md

@@ -286,6 +286,8 @@ Minor releases are done differently from major releases: They are built off of l
 6. Create a pull request back to master with the CHANGELOG & version changes from the latest release.
    - Remove all `R:minor` labels from the pull requests that were included in the release.
    - Do not merge the release branch into master.
+7. Delete the former long lived release candidate branch once the release has been made.
+8. Create a new release candidate branch to be used for the next release.
 
 #### Backport Release