ci: Automate weekly proxy releases

This commit adds a release-weekly workflow that runs on Wednesday nights. If
there hasn't been a proxy release earlier in the week and there are new commits
on main, then a new release is triggered.

.github/workflows/release-weekly.yml

@@ -0,0 +1,78 @@
+name: Weekly proxy release
+
+on:
+  schedule:
+    # Wednesday at ~8:10PM Pacific
+    - cron: "10 3 * * 3"
+  workflow_dispatch: {}
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref }}
+  cancel-in-progress: true
+
+jobs:
+  last-release:
+    if: github.repository == 'linkerd/linkerd2-proxy' # Don't run this in forks.
+    runs-on: ubuntu-22.04
+    timeout-minutes: 5
+    env:
+      GH_REPO: ${{ github.repository }}
+      GH_TOKEN: ${{ github.token }}
+    steps:
+      - name: Latest release
+        id: latest
+        run: gh release view --json name,publishedAt | jq -r 'to_entries[] | (.key + "=" + .value)' >> "$GITHUB_OUTPUT"
+      - name: Check if release was in last 72 hours
+        id: recency
+        env:
+          PUBLISHED_AT: ${{ steps.latest.outputs.publishedAt }}
+        run: |
+          if [ "$(date -d "$PUBLISHED_AT" +%s)" -gt "$(date -d '72 hours ago' +%s)" ]; then
+            echo "Last release $PUBLISHED_AT is recent" >&2
+            echo "recent=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "Last release $PUBLISHED_AT is not recent" >&2
+            echo "recent=false" >> "$GITHUB_OUTPUT"
+          fi
+    outputs:
+      version: ${{ steps.latest.outputs.name }}
+      published-at: ${{ steps.latest.outputs.publishedAt }}
+      recent: ${{ steps.recency.outputs.recent == 'true' }}
+
+  last-commit:
+    needs: last-release
+    runs-on: ubuntu-22.04
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@9bb56186c3b09b4f86b1c65136769dd318469633
+      - name: Check if the most recent commit is after the last release
+        id: recency
+        env:
+          PUBLISHED_AT: ${{ needs.last-release.outputs.published-at }}
+        run: |
+          if [ "$(git log -1 --format=%ct)" -gt "$(date -d "$PUBLISHED_AT" +%s)" ]; then
+            echo "HEAD after last release $PUBLISHED_AT" >&2
+            echo "after-release=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "after-release=false" >> "$GITHUB_OUTPUT"
+          fi
+    outputs:
+      after-release: ${{ steps.recency.outputs.after-release == 'true' }}
+
+  trigger-release:
+    needs: [last-release, last-commit]
+    if: needs.last-release.outputs.recent == 'false' && needs.last-commit.outputs.after-release == 'true'
+    runs-on: ubuntu-22.04
+    timeout-minutes: 5
+    permissions:
+      actions: write
+    env:
+      GH_REPO: ${{ github.repository }}
+      GH_TOKEN: ${{ github.token }}
+      LAST_VERSION: ${{ needs.last-release.outputs.version }}
+    steps:
+      - name: Get the latest minor version
+        run: |
+          m="$(echo "$LAST_VERSION" | cut -d. -f2)"
+          echo MINOR_VERSION="$((m+1))" >> "$GITHUB_ENV"
+      - run: gh workflow run release.yml -f publish=true -f version=v2."$MINOR_VERSION".0