Use JDK 11 for bootstrap build

which is necessary to enable the use of Java 11 language features.

PiperOrigin-RevId: 422598120
diff --git a/scripts/bootstrap/bootstrap.sh b/scripts/bootstrap/bootstrap.sh
index 597379b..2422236 100755
--- a/scripts/bootstrap/bootstrap.sh
+++ b/scripts/bootstrap/bootstrap.sh
@@ -29,7 +29,7 @@
     EMBED_LABEL_ARG=(--stamp --embed_label "${EMBED_LABEL}")
 fi
 
-: ${JAVA_VERSION:="1.8"}
+: ${JAVA_VERSION:="11"}
 
 _BAZEL_ARGS="--spawn_strategy=standalone \
       --nojava_header_compilation \