chore: enable release-trigger bot (#452)

Source-Author: Jeff Ching <chingor@google.com>
Source-Date: Tue Aug 24 15:30:40 2021 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 63cff634aabb85854caa511c5837ea6b45f42b4b
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-shared-dependencies.git",
-        "sha": "8ed008c2a580785896c8e94a85a709c95ce75440"
+        "sha": "b28723ace0c27ebc73f23a2d094e0fe9ba8a9075"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ff01716e16d2c6e87eaf87197b753ac9fcbbed5d"
+        "sha": "63cff634aabb85854caa511c5837ea6b45f42b4b"
       }
     }
   ],
@@ -24,6 +24,7 @@
     ".github/blunderbuss.yml",
     ".github/generated-files-bot.yml",
     ".github/readme/synth.py",
+    ".github/release-trigger.yml",
     ".github/snippet-bot.yml",
     ".github/sync-repo-settings.yaml",
     ".github/trusted-contribution.yml",