Update EBCLI version for 3.21 release

CHANGES.rst

@@ -1,6 +1,20 @@
 
 Changelog
 =========
+--------------------
+3.21 (2024-09-17)
+--------------------
+- Updated botocore requirement to `>=1.35.0,<1.36.0`
+- Updated cement to `2.10.14` to eable EBCLI to work with Python 3.12
+- Updated colorama requirement to `>=0.4.6,<0.5`
+- Updated semantic_version requirement to `>=2.10.0,<2.11`
+- Updated termcolor requirement to `>=2.4.0,<3`
+- Updated wcwidth requirement to `>=0.2.13,<0.3`
+- Updated blessed requirement from Windows to `>=1.20.0`
+- Declared support drop for Python 2.7 and Python < 3.7
+- Fixed bug preventing environment creates and updates with JSON ebextensions
+- Removed explicitly checking for Docker installation
+
 --------------------
 3.20.10 (2023-09-15)
 --------------------