chore: enable release-trigger bot (#1387)

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Source-Link: googleapis/synthtool@63cff63

Author: yoshi-automation

.github/release-trigger.yml

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

synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "2d605e405c9efad5b18f66eff9e5c9942413404f"
+        "sha": "2ea0e645ec20c29c2bd409bd3ec733b0a2c0a7f1"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ff01716e16d2c6e87eaf87197b753ac9fcbbed5d"
+        "sha": "63cff634aabb85854caa511c5837ea6b45f42b4b"
       }
     }
   ],
@@ -59,6 +59,7 @@
     ".github/PULL_REQUEST_TEMPLATE.md",
     ".github/generated-files-bot.yml",
     ".github/readme/synth.py",
+    ".github/release-trigger.yml",
     ".github/snippet-bot.yml",
     ".github/trusted-contribution.yml",
     ".github/workflows/approve-readme.yaml",