release: prepare to release v0.21.0-rc1

This should be followed shortly by v0.21.0, ideally with no further changes.
But since this is the first release for nearly 2 years it seems prudent to
be a little cautious and publish an actual release candidate first.

bump version to v0.20.0

Bump version to v0.19.1

Update version to v0.19.0

Preparing for v0.18.0

Release 0.17.0

Release v0.16.0

Release 0.15.0.

Release 0.14.0

Bump version to v0.13.0