Publish to Maven Central using GitHub Actions

[tumbarumba]

As per the title, I'm creating an issue to track the work of automatically publishing Hamcrest through GitHub actions. There was already much discussion on #405, but this deserves its own issue. @olibye is assigned, but if I (@tumbarumba) can work out what to do, I can probably help too.

We may be able to copy parts of the jMock release workflow (see https://github.com/jmock-developers/jmock-library/blob/master/.github/workflows/release.yml), though jMock uses Maven, so is probably not directly transferable to a Gradle project