ci/release: Use latest Go, support manual run

Use the latest Go (this is needed now),
and support manually invoking the release workflow
for failed partial releases.

.github/workflows/release.yml

@@ -4,6 +4,13 @@ on:
   push:
     tags: ['v*']
 
+  workflow_dispatch:
+    inputs:
+      version:
+        description: "Version to release, including the 'v' prefix."
+        required: true
+        type: string
+
 permissions:
   contents: write
 
@@ -13,11 +20,13 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
+      with:
+        ref: ${{ inputs.version || github.ref }}
 
     - name: Set up Go
       uses: actions/setup-go@v4
       with:
-        go-version: 1.20.x
+        go-version: 1.21.x
         cache: true
 
     - name: Install parse-changelog
@@ -30,12 +39,18 @@ jobs:
       with:
         fetch-depth: 0
 
-    - name: Determine version
+    - name: Determine version (tagged release)
       if:  github.event_name == 'push' && startsWith(github.ref, 'refs/tags/v')
       run: |
         set -eou pipefail
         REF=${{ github.ref }}
         echo "VERSION=${REF#refs/tags/v}" >> "$GITHUB_ENV"
+    - name: Determine version (manual dispatch)
+      if:  github.event_name == 'workflow_dispatch'
+      run: |
+        echo "VERSION=${INPUT_VERSION#v}" >> "$GITHUB_ENV"
+      env:
+        INPUT_VERSION: ${{ inputs.version }}
 
     - name: Extract changelog
       run: |