Simplify the release process by linking to release notes instead of copy/pasting them

[jeffwidman]

There's a chicken-and-egg scenario where we don't have the release notes to include in the version bump PR until we create a GitHub release... but we don't want to publish the release until the commit bumping the version actually lands.

I don't think the juice is worth the squeeze... no one really looks at the changelog notes in the PR, and if in fact having the release notes two different places introduces a risk of drift because both the PR description and the git tag for the release can be edited later on... only the commit history is actually immutable. So if either is edited w/o editing the other, they're out of sync.

The odds of that are low--we rarely edit release notes--but still life is simpler if we merely point the commit/PR description at the URL for the release notes and manage those in a single place.

This also lets us script creating the PR, which is one less thing to do manually.

[jeffwidman]

We could automate this as well, so that anytime a new v1.x version is tagged, the v1 tracking tag is updated to point to it. Or is that too much "magic"??