Add GitHub template and workflows needed on the default branch (mmtk#70)

* Copy across GitHub Action workflows

* Add workflow to close stale PRs

* Add PR template

* Remove codeowners from upstream

.github/pull_request_template.md

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+<!---
+PRs to RelationalAI/julia must be opened to the correct branch (see
+https://github.com/RelationalAI/raicode/blob/master/nix/julia-version.json).
+-->
+## PR Description
+
+_What does this PR do?_
+
+## Checklist
+
+Requirements for merging:
+- [ ] I have opened an issue or PR upstream on JuliaLang/julia: <link to JuliaLang/julia>
+- [ ] I have removed the `port-to-*` labels that don't apply.

.github/workflows/LabelCheck.yml

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+name: Labels
+
+permissions:
+  contents: read
+on:
+  pull_request:
+    types: [labeled, unlabeled, opened, reopened, edited, synchronize]
+jobs:
+  enforce-labels:
+    name: Check for blocking labels
+    runs-on: ubuntu-latest
+    timeout-minutes: 2
+    steps:
+    - uses: yogevbd/enforce-label-action@2.2.2
+      with:
+        # REQUIRED_LABELS_ANY: "bug,enhancement,skip-changelog"
+        # REQUIRED_LABELS_ANY_DESCRIPTION: "Select at least one label ['bug','enhancement','skip-changelog']"
+        BANNED_LABELS: "needs docs,needs compat annotation,needs more info,needs nanosoldier run,needs news,needs pkgeval,needs tests,DO NOT MERGE"
+        BANNED_LABELS_DESCRIPTION: "A PR should not be merged with `needs *` or `DO NOT MERGE` labels"

.github/workflows/labeler.yml

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+# See https://github.com/actions/labeler
+name: "Pull Request Labeler"
+on:
+  pull_request_target:
+    types:
+      - opened
+
+jobs:
+  triage:
+    permissions:
+      contents: read
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/labeler@v4
+      with:
+        dot: true

.github/workflows/stale.yml

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+name: "Close stale PRs"
+on:
+  schedule:
+  - cron: "0 0 * * *" # every night at midnight
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/stale@v8
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-pr-message: 'This PR is stale because it has been open 30 days with no activity. Comment or remove stale label, or this PR will be closed in 5 days.'
+        days-before-stale: 30
+        days-before-close: 5
+        stale-pr-label: 'stale'