misc: Updated Doxygen to state version as v19.0.0.0

Change-Id: I6baf60e1f59c0e856c2184fcb0211ed85a62a794
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25363
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/src/Doxyfile b/src/Doxyfile
index 891720a..5b18e97 100644
--- a/src/Doxyfile
+++ b/src/Doxyfile
@@ -31,7 +31,7 @@
 # This could be handy for archiving the generated documentation or 
 # if some version control system is used.
-PROJECT_NUMBER         = 
+PROJECT_NUMBER         = v19.0.0.0
 # The OUTPUT_DIRECTORY tag is used to specify the (relative or absolute) 
 # base path where the generated documentation will be put.