Create issue every July to update Ubuntu version (#55)

.github/ISSUE_TEMPLATE/update-ubuntu-version-in-github-actions.md

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+---
+name: Update Ubuntu version in GitHub Actions
+about: Stay up to date with Ubuntu
+title: Update Ubuntu version in GitHub Actions
+labels: ''
+assignees: ''
+
+---
+
+[Ubuntu releases annually in April](https://wiki.ubuntu.com/Releases).  
+
+In 2020 the GitHub Actions team [supported the April release for that year by mid June](https://github.com/actions/virtual-environments/issues/228#issuecomment-644065532), so this GitHub Issue gets automatically created annually each year on 15 July for us to do the update (as hopefully GitHub Actions will support the new version by then each year).  
+
+We can find out if we can update yet [here](https://docs.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners#supported-runners-and-hardware-resources).  
+
+When we do the update to the new version it involves e.g. for 2021, simply replacing every case of `ubuntu-20.04` with `ubuntu-21.04`.

.github/workflows/schedule_ubuntu_annual_update_issue.yml

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+---
+name: Create issue annually to update Ubuntu
+on:  # yamllint disable-line rule:truthy
+  # Scheduled for 2am on 15th July every year
+  schedule:
+    - cron: '0 2 15 7 *'  # * is a special character in YAML so we have to quote this string
+
+jobs:
+  create_issue:
+    name: Create issue to update Ubuntu
+    runs-on: ubuntu-20.04
+    steps:
+
+      # Repo code checkout required if `template` is used
+      - name: Checkout
+        uses: actions/[email protected]
+
+      - name: issue-bot
+        uses: imjohnbo/[email protected]
+        with:
+          assignees: "johnboyes"
+          labels: "dependencies"
+          # assignees & labels in the template are overridden by the values specified in this action
+          template: ".github/ISSUE_TEMPLATE/update-ubuntu-version-in-github-actions.md"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}