chore: update gapic-generator-java to 0.0.18 (#370)

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PiperOrigin-RevId: 350949863
Source-Link: googleapis/googleapis@91e206b

synth.metadata

@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "6133907dbb3ddab204a17a15d5c53ec0aae9b033"
+        "sha": "fb53b6fb373b7c3edf4e55f3e8036bc6d73fa483"
       }
     }
   ],