Update Go on the bots.

Change-Id: Id49fb3e73e6ada87feaa621c555924efbaf672f2
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/43505
Reviewed-by: Adam Langley <agl@google.com>

util/bot/go/bootstrap.py

@@ -45,7 +45,7 @@ WORKSPACE = os.path.join(ROOT, 'go')
 EXE_SFX = '.exe' if sys.platform == 'win32' else ''
 
 # Pinned version of Go toolset to download.
-TOOLSET_VERSION = 'go1.14.1'
+TOOLSET_VERSION = 'go1.15.3'
 
 # Platform dependent portion of a download URL. See http://golang.org/dl/.
 TOOLSET_VARIANTS = {