MNT Add merge-up and keepalive workflows

.github/workflows/keepalive.yml

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+name: Keepalive
+
+on:
+  # At 1:05 PM UTC, on day 22 of the month
+  schedule:
+    - cron: '5 13 22 * *'
+  workflow_dispatch:
+
+jobs:
+  keepalive:
+    name: Keepalive
+    # Only run cron on the silverstripe account
+    if: (github.event_name == 'schedule' && github.repository_owner == 'silverstripe') || (github.event_name != 'schedule')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Keepalive
+        uses: silverstripe/gha-keepalive@v1

.github/workflows/merge-up.yml

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+name: Merge-up
+
+on:
+  # At 2:20 PM UTC, only on Saturday
+  schedule:
+    - cron: '20 14 * * 6'
+  workflow_dispatch:
+
+jobs:
+  merge-up:
+    name: Merge-up
+    # Only run cron on the silverstripe account
+    if: (github.event_name == 'schedule' && github.repository_owner == 'silverstripe') || (github.event_name != 'schedule')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Merge-up
+        uses: silverstripe/gha-merge-up@v1