Migrate all workflows to GitHub Actions

This commit completes the migration from Concourse to GitHub Actions,
adding the CI variants and release workflows.

See gh-1024

.github/actions/create-github-release/action.yml

@@ -0,0 +1,23 @@
+name: Create GitHub Release
+description: Create the release on GitHub with a changelog
+inputs:
+  milestone:
+    description: 'Name of the GitHub milestone for which a release will be created'
+    required: true
+  token:
+    description: 'Token to use for authentication with GitHub'
+    required: true
+runs:
+  using: composite
+  steps:
+    - name: Generate Changelog
+      uses: spring-io/github-changelog-generator@185319ad7eaa75b0e8e72e4b6db19c8b2cb8c4c1 #v0.0.11
+      with:
+        milestone: ${{ inputs.milestone }}
+        token: ${{ inputs.token }}
+        config-file: .github/actions/create-github-release/changelog-generator.yml
+    - name: Create GitHub Release
+      env:
+        GITHUB_TOKEN: ${{ inputs.token }}
+      shell: bash
+      run: gh release create ${{ format('v{0}', inputs.milestone) }} --notes-file changelog.md

.github/actions/create-github-release/changelog-generator.yml

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+changelog:
+  repository: spring-projects/spring-graphql
+  sections:
+    - title: ":star: New Features"
+      labels:
+        - "type: enhancement"
+    - title: ":lady_beetle: Bug Fixes"
+      labels:
+        - "type: bug"
+        - "type: regression"
+    - title: ":notebook_with_decorative_cover: Documentation"
+      labels:
+        - "type: documentation"
+    - title: ":hammer: Dependency Upgrades"
+      sort: "title"
+      labels:
+        - "type: dependency-upgrade"
+  contributors:
+    exclude:
+      names:
+        - "bclozel"
+        - "rstoyanchev"