Integrate renovate bot for all dependency updates (#37050)

Replaces Dependabot with Renovate. The new setup:

- One PR per ecosystem (GitHub Actions, Go modules + Makefile go-tool
pins, npm, Python via uv, Nix flake), opened weekly on Mondays with a
5-day release-age cooldown. Vulnerability PRs ship next-day via daily
cron + Renovate's `vulnerabilityAlerts` schedule bypass.
- All `uses:` action refs SHA-pinned with patch-level version comments
(same format as #36971, which this supersedes);
`helpers:pinGitHubActionDigests` keeps future bumps in that format.
- `renovatebot/github-action` runtime image pinned via the
upstream-recommended `RENOVATE_VERSION` env + magic comment +
`customManagers:githubActionsVersions` preset, so Renovate keeps the pin
updated.
- Custom regex manager tracks the `*_PACKAGE ?= <import-path>@<version>`
lines in `Makefile` (golangci-lint, swagger, actionlint, etc.) and
groups them into the same Go PR via `matchDatasources: ["go"]`.
- Post-upgrade tasks regenerate `assets/go-licenses.json` (`make tidy`)
and the SVG sprite (`make svg`), gated by an env-level command
allowlist.
- Replaces the standalone `cron-flake-updater` workflow — Renovate's nix
manager tracks `flake.nix` inputs and produces the same `flake.lock`
bump PRs on the regular weekly schedule.
- npm and gomod-replace pins live in `renovate.json5` only;
`updates@17.16.3` reads them from there too, so the standalone
`updates.config.ts` is gone and one source of truth covers both tools.

Fixes: https://github.com/go-gitea/gitea/issues/33386
Signed-off-by: silverwind <me@silverwind.io>
Signed-off-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: Giteabot <teabot@gitea.io>

.github/workflows/pull-e2e-tests.yml

@@ -20,8 +20,8 @@ jobs:
     permissions:
       contents: read
     steps:
-      - uses: actions/checkout@v6
-      - uses: actions/setup-go@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
         with:
           go-version-file: go.mod
           check-latest: true
@@ -30,8 +30,8 @@ jobs:
         with:
           cache-name: e2e
           build-cache: "false"
-      - uses: pnpm/action-setup@v5
-      - uses: actions/setup-node@v6
+      - uses: pnpm/action-setup@fc06bc1257f339d1d5d8b3a19a8cae5388b55320 # v5.0.0
+      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
         with:
           node-version: 24
           cache: pnpm