Dropped Node 16 from CI matrix tests

refs https://github.com/TryGhost/DevOps/issues/75

- Node 16 has gone EOL so we can from support for it from our matrix
  tests
- dropping support in some other tests and in general will come in
  future commits

.github/workflows/ci.yml

@@ -263,7 +263,7 @@ jobs:
     if: needs.job_get_metadata.outputs.changed_any_code == 'true'
     strategy:
       matrix:
-        node: [ '16.14.0', '18.12.1' ]
+        node: [ '18.12.1' ]
     name: Unit tests (Node ${{ matrix.node }})
     steps:
       - uses: actions/checkout@v4
@@ -301,7 +301,7 @@ jobs:
     if: needs.job_get_metadata.outputs.changed_core == 'true'
     strategy:
       matrix:
-        node: [ '16.14.0', '18.12.1' ]
+        node: [ '18.12.1' ]
         env:
           - DB: mysql8
             NODE_ENV: testing-mysql