Bump to 1.21.0

- New support for creating "simple signing" signatures using Sequoia-PGP,
  dependent on a build tag that enables it
- New option (skopeo copy --force-compression-format)
- New option --user-agent-prefix
- TLS options on the command line of (skopeo sync) take precedence
  over options in YAML

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

version/version.go

@@ -1,4 +1,4 @@
 package version
 
 // Version is the version of the build.
-const Version = "1.21.0-dev"
+const Version = "1.21.0"