Update dev branch in release script

I forgot that one can actually use the script to branch, so manually updating it.

Change-Id: I36a91894378eaf6ec08e4db591ca8cb7c0cd6d10
diff --git a/tools/r8_release.py b/tools/r8_release.py
index a694ae0..f74fd57 100755
--- a/tools/r8_release.py
+++ b/tools/r8_release.py
@@ -19,7 +19,7 @@
 import update_prebuilds_in_android
 import utils
 
-R8_DEV_BRANCH = '2.0'
+R8_DEV_BRANCH = '2.1'
 R8_VERSION_FILE = os.path.join(
     'src', 'main', 'java', 'com', 'android', 'tools', 'r8', 'Version.java')
 THIS_FILE_RELATIVE = os.path.join('tools', 'r8_release.py')