MNT Use GitHub Actions CI

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+  # Every Thursday at 12:20pm UTC
+  schedule:
+    - cron: '20 12 * * 4'
+
+jobs:
+  ci:
+    name: CI
+    # Only run cron on the silverstripe account
+    if: (github.event_name == 'schedule' && startsWith(github.repository, 'silverstripe/')) || (github.event_name != 'schedule')
+    uses: silverstripe/gha-ci/.github/workflows/ci.yml@v1

.github/workflows/keepalive.yml

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+name: Keepalive
+
+on:
+  workflow_dispatch:
+  # The 4th of every month at 10:50am UTC
+  schedule:
+    - cron: '50 10 4 * *'
+
+jobs:
+  keepalive:
+    name: Keepalive
+    # Only run cron on the silverstripe account
+    if: (github.event_name == 'schedule' && startsWith(github.repository, 'silverstripe/')) || (github.event_name != 'schedule')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Keepalive
+        uses: silverstripe/gha-keepalive@v1