Properly maintain milestones

[ktoso]

Expected behavior

Milestones should be in sync with released versions.

Actual behavior

We released https://github.com/apple/swift-log/releases 1.1.1 but no such milestone exists.

1.1.0 exist and is not closed. etc.

Proposal

When closing tickets, always assign a milestone;
When releasing a version, ensure tickets or PRs if no ticket present for PR are assigned to milestone; Close milestone, link to it from release notes.

I think it matters a lot to be clean and consistent here.

We should start with cleaning up the existing issues / releases.
I can take this on.

WDYT @tomerd @weissi