chore: enable release-trigger bot (#536)

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Source-Link: googleapis/synthtool@63cff63

.github/release-trigger.yml

@@ -0,0 +1 @@
+enabled: true

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-core.git",
-        "sha": "55f19b5095602e39b153fb55feb8c0ab33737944"
+        "sha": "c74cb60857c51f78193f553885307d678cd32340"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ff01716e16d2c6e87eaf87197b753ac9fcbbed5d"
+        "sha": "63cff634aabb85854caa511c5837ea6b45f42b4b"
       }
     }
   ],
@@ -25,6 +25,7 @@
     ".github/generated-files-bot.yml",
     ".github/readme/synth.py",
     ".github/release-please.yml",
+    ".github/release-trigger.yml",
     ".github/snippet-bot.yml",
     ".github/sync-repo-settings.yaml",
     ".github/trusted-contribution.yml",