Run release workflow after CI workflow

With the current configuration, the release workflow and the CI
workflow run in parallel. We would want the CI workflow to finish
before the release workflow runs. With this PR:

- The release workflow will run when a CI workflow completes in `main`
- The release job in the workflow will only run if:
  - The CI workflow was successful, and
  - The head commit message for that workflow contains "Prepare release v"
- A version check exits the job if version in `plugin.yaml` has not changed
from the previous release (the latest tag equals the version in the file)

.github/workflows/release.yaml

@@ -1,26 +1,36 @@
 name: Release
 on:
-  pull_request_target:
+  workflow_run:
+    workflows:
+      - CI
     types:
-      - closed
-    paths:
-      - 'plugin.yaml'
+      - completed
+    branches:
+      - main
 
 permissions:
   contents: write
 
 jobs:
   Release:
-    if: (github.event.pull_request.merged == true && startsWith(github.head_ref, 'release/'))
     runs-on: ubuntu-latest
+    if: ${{ github.event.workflow_run.conclusion == 'success' && contains(github.event.workflow_run.head_commit.message, 'Prepare release v') }}
     steps:
       - name: Checkout
         uses: actions/checkout@c85c95e3d7251135ab7dc9ce3241c5835cc595a9 # v3
         with:
           fetch-depth: 0
 
       - name: Fetch Version
-        run: echo PLUGIN_VERSION=v$(cat plugin.yaml | grep "version" | cut -d '"' -f 2) >> "$GITHUB_ENV"
+        run: |
+          PLUGIN_VERSION=v$(cat plugin.yaml | grep "version" | cut -d '"' -f 2)
+          LATEST_VERSION=$(git describe --tags --abbrev=0)
+          echo PLUGIN_VERSION=$PLUGIN_VERSION >> "$GITHUB_ENV"
+          echo LATEST_VERSION=$LATEST_VERSION >> "$GITHUB_ENV"
+
+      - name: Check Version
+        if: ${{ env.PLUGIN_VERSION == env.LATEST_VERSION }}
+        run: echo "Plugin version already released. Please make sure you have prepared the release first." && exit 1
 
       - name: Set Golang
         uses: actions/setup-go@fac708d6674e30b6ba41289acaab6d4b75aa0753 # v4.0.1