Update CHANGELOG.md links for calver (#287)

This PR updates the `0.20` references in `CHANGELOG.md` to be `21.06`.

Authors:
  - AJ Schmidt (https://github.com/ajschmidt8)

Approvers:
  - Dillon Cullinan (https://github.com/dillon-cullinan)

URL: #287

CHANGELOG.md

@@ -1,6 +1,6 @@
-# cuXfilter 0.20.0 (Date TBD)
+# cuXfilter 21.06.00 (Date TBD)
 
-Please see https://github.com/rapidsai/cuxfilter/releases/tag/v0.20.0a for the latest changes to this development branch.
+Please see https://github.com/rapidsai/cuxfilter/releases/tag/v21.06.00a for the latest changes to this development branch.
 
 # cuXfilter 0.19.0 (21 Apr 2021)