Prepare changelog for 1.21.0 release

Author: carmstrong

CHANGELOG.md

@@ -2,6 +2,13 @@ Java Cookbook CHANGELOG
 =======================
 This file is used to list changes made in each version of the Java cookbook.
 
+v1.21.0
+-------
+### Improvement
+- **[#143](https://github.com/socrata-cookbooks/java/pull/143)** - Symlink /usr/lib/jvm/default-java for both OpenJDK and Oracle
+- **[#144](https://github.com/socrata-cookbooks/java/pull/144)** - Remove /var/lib/alternatives/#{cmd} before calling alternatives (Hopefully fixes sporadic issues when setting alternatives)
+- **[Make default_java_symlink conditional on set_default attribute](https://github.com/socrata-cookbooks/java/commit/e300e235a463382a5022e1dddaac674930b4d138)**
+
 v1.20.0
 -------
 ### Improvement