ci: update toolchains and let renovate update action inputs (#40092)

- Let renovate update hyphenated action inputs like `python-version`
- Python 3.14 → 3.15 in CI
- Download gh 2.103.2 directly in the release workflow instead of using
`install-gh-cli-action` with 2.39.1
- Devcontainer Go image 1.26 → 1.27
- Snap build toolchains Go 1.26 → 1.27 and Node 24 → 26

.devcontainer/devcontainer.json

@@ -1,6 +1,6 @@
 {
   "name": "Gitea DevContainer",
-  "image": "mcr.microsoft.com/devcontainers/go:1.26-trixie",
+  "image": "mcr.microsoft.com/devcontainers/go:1.27-trixie",
   "containerEnv": {
     // override "local" from packaged version
     "GOTOOLCHAIN": "auto"