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/changelog-generator.yml

@@ -0,0 +1,22 @@
+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"