deps: bump minimum ICU version to 68

Refs: v8/v8@1e69cdd

PR-URL: #37330
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

tools/icu/icu_versions.json

@@ -1,3 +1,3 @@
 {
-    "minimum_icu": 67
+    "minimum_icu": 68
 }