TensorBoard 2.21.0 (#7130)

This is the 3/3 changes for release of 2.21.0

Author: psamanoelton

tensorboard/version.py

@@ -15,7 +15,7 @@
 
 """Contains the version string."""
 
-VERSION = "2.21.0a0"
+VERSION = "2.21.0"
 
 if __name__ == "__main__":
     print(VERSION)