chore(ci): migrate release pipeline from release-please App to GitHub Actions

Remove the release-please GitHub App config files and add an Actions-based
release pipeline with candidate branch strategy. This includes workflows for
cutting releases, running release-please, finalizing releases, publishing to
PyPI, and cherry-picking fixes to release candidates.

Co-authored-by: Wei Sun (Jack) <weisun@google.com>
PiperOrigin-RevId: 868825704

.github/release-please-config.json

@@ -0,0 +1,24 @@
+{
+  "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
+  "packages": {
+    ".": {
+      "release-type": "python",
+      "package-name": "google-adk",
+      "include-component-in-tag": false,
+      "skip-github-release": true,
+      "changelog-path": "CHANGELOG.md",
+      "changelog-sections": [
+        {"type": "feat", "section": "Features"},
+        {"type": "fix", "section": "Bug Fixes"},
+        {"type": "perf", "section": "Performance Improvements"},
+        {"type": "refactor", "section": "Code Refactoring"},
+        {"type": "docs", "section": "Documentation"},
+        {"type": "test", "section": "Tests", "hidden": true},
+        {"type": "build", "section": "Build System", "hidden": true},
+        {"type": "ci", "section": "CI/CD", "hidden": true},
+        {"type": "style", "section": "Styles", "hidden": true},
+        {"type": "chore", "section": "Miscellaneous Chores", "hidden": true}
+      ]
+    }
+  }
+}