misc: Update version number to v21.0.0.0

Change-Id: Ica7f7bfdcb1655e38defdf0b32a630e90adb7ce8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43405
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/Doxyfile

@@ -31,7 +31,7 @@ PROJECT_NAME           = gem5
 # This could be handy for archiving the generated documentation or 
 # if some version control system is used.
 
-PROJECT_NUMBER         = DEVELOP-FOR-V20.2
+PROJECT_NUMBER         = v21.0.0.0
 
 # The OUTPUT_DIRECTORY tag is used to specify the (relative or absolute) 
 # base path where the generated documentation will be put. 

src/base/version.cc

@@ -29,4 +29,4 @@
 /**
  * @ingroup api_base_utils
  */
-const char *gem5Version = "[DEVELOP-FOR-V20.2]";
+const char *gem5Version = "21.0.0.0";