Added Node 20 to CI matrix

ref https://linear.app/tryghost/issue/ENG-765/add-support-for-node-20

- Node 20 is LTS so we should add support for it and test against this
  version

Author: daniellockyer

.github/workflows/lint-only.yml

@@ -7,7 +7,7 @@ jobs:
     if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
     strategy:
       matrix:
-        node: [ 18 ]
+        node: [ 18, 20 ]
     env:
       FORCE_COLOR: 1
     name: Node ${{ matrix.node }}

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
     if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
     strategy:
       matrix:
-        node: [ 18 ]
+        node: [ 18, 20 ]
     env:
       FORCE_COLOR: 1
     name: Node ${{ matrix.node }}