Updated actions/trigger-metric to Node 20

- fixes warning around: `The following actions use a deprecated Node.js version and will be forced to run on node20: tryghost/actions/actions/trigger-metric@main.`

Author: daniellockyer

actions/trigger-metric/action.yml

@@ -1,7 +1,7 @@
 name: 'Ghost Metric Shipper'
 description: 'Send metrics to various backends using @tryghost/metrics for GitHub Actions'
 runs:
-  using: 'node16'
+  using: 'node20'
   main: 'dist/index.js'
 inputs:
   metricName: