Soy: bump version and use new "??" operator

This is a part of a LSC by the Soy team to converge Soy with TypeScript syntax.

In order to fix this, we will update the Soy version to include the new
"??" syntax and update any pre-existing usage from "?:" to "??". We also
have to update deltemplate to modifiable templates.

Change-Id: I5dff1ea5b2d9025153034812d6ec0f0282e1643e

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 0c55d21..b1bedac 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -140,8 +140,8 @@
 
 maven_jar(
     name = "soy",
-    artifact = "com.google.template:soy:2022-07-20",
-    sha1 = "f64eb90da6d91beddf11653865c90f26d26710cf",
+    artifact = "com.google.template:soy:2023-09-13",
+    sha1 = "b88a029df7408c58452f80d6518723f1386d69b5",
 )
 
 FLOGGER_VERS = "0.7.4"