Add automated reminder for API updates

Author: andyw8

.github/api_update_reminder.md

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+---
+title: A new release of the Shopify API is due soon
+assignees: andyw8 # @platform/dev-tools-education
+labels: automated
+---
+
+This is an automated reminder for the maintainers that a new Stable release of the Shopify API is due soon, so the library needs to be updated.
+
+A new library release should be prepared to:
+* add the upcoming Stable release
+* remove the oldest Stable release which will no longer be supported.
+
+The PR should be created as a **draft** but not yet be merged.
+
+The release schedule can be found at https://shopify.dev/concepts/about-apis/versioning
+
+Review the changelog and consider if anything in the library needs to change:
+
+https://shopify.dev/changelog
+
+Test against the upcoming release by using the Release Candidate.
+
+Another reminder issue will be created on the date of the next release.
+When that happens, test again using the Stable API version, and aim to release an update of the library within one week.
+
+Thank you!

.github/workflows/api_update_reminder.yml

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+on:
+  workflow_dispatch: # to allow for manual testing
+  # schedule:
+  #   - cron: "0 0 1 3,6,9,12 *" # At 00:00 on 1st of March, June, September, and December
+
+name: API update reminder
+jobs:
+  reminder:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: JasonEtco/[email protected]
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          filename: .github/api_update_reminder.md