Adding release checksum.

Signing v0.22.0

Added tag v0.22.0 for changeset 66dc3cb02fc4

Bump the minor version, update history and gemspec.

Make Sixfish the default RDoc generator

Fix diffing for the history file under Git

Adding release checksum.

Signing v0.21.0

Added tag v0.21.0 for changeset ce3ec0e563c1

Bump the minor version, update history.

Allow declaration of alternative RDoc generator

Only include plain files in the manifest

Adding release checksum.

Signing v0.20.0

Added tag v0.20.0 for changeset e308d450ce45

Bump the minor version, update history

Fix missing default and allow default manifest to be modified

Bump default Ruby version to 3.1

Update gem-signing cert

Update dependencies.