Bump tag to rc1

Signed-off-by: Brian L. Troutwine <[email protected]>

Author: blt

CHANGELOG.md

@@ -6,7 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
-## [0.21.0-rc0]
+## [0.21.0-rc1]
 ### Added
 ### Changed
 - Maximum datagram size in bytes for unix datagram generator is 8,192.