Add GitHub workflow that automatically updates description of a release PR

[mcmire]

One of the steps involved in approving a release is to look over the dry-run output. Currently this output is not as helpful as it could be as it lists all packages instead of only the ones that are being released, so we need to also take a look at the release PR itself to confirm which packages are being released. The way to do this is to go through the PR and check which packages are being bumped, but it would be easier if we didn't have to do this and if we could simply check the PR description instead. It would be even easier if the description for a release PR was updated automatically so that it always reflected the contents of the PR.