[release] Increase version on main to 3.13

Change-Id: Ice6dfd3b694637da4e20529af9374fece32eda4c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/491821
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

tools/VERSION

@@ -25,7 +25,7 @@
 #
 CHANNEL main
 MAJOR 3
-MINOR 12
+MINOR 13
 PATCH 0
 PRERELEASE 0
 PRERELEASE_PATCH 0

tools/experimental_features.yaml

@@ -126,7 +126,7 @@
 # default 'language' "category" with code generated for both CFE and Analyzer,
 # while other categories can be tailored more specifically.
 
-current-version: '3.12.0'
+current-version: '3.13.0'
 
 features:
   variance: