Rename act_runner to runner (#850)

## Consumer-facing breaking changes

- **Go module path**: `gitea.com/gitea/act_runner` → `gitea.com/gitea/runner`. Anything importing `act/...` or `internal/...` packages (notably Gitea itself) must update imports.
- **Binary name**: `act_runner` → `gitea-runner`. Wrapper scripts, systemd units, init scripts, and documentation referencing the binary by `act_runner` will break.
- **Docker image**: `gitea/act_runner` → `gitea/runner` (incl. `*-dind-rootless` variants). Users pulling `gitea/act_runner:nightly` etc. will get stale images. Note: the image name is `gitea/runner`, not `gitea/gitea-runner`.
- **Release artifact paths**: S3 directory `act_runner/{{.Version}}` → `gitea-runner/{{.Version}}`, and artifact filenames change with the new project name. Existing download URLs break.
- **Metrics namespace**: changed from `act_runner` to `gitea_runner` (e.g. `act_runner_jobs_total` → `gitea_runner_jobs_total`); existing monitors/dashboards must be updated.
- **ldflags version path**: `gitea.com/gitea/act_runner/internal/pkg/ver.version` → `gitea.com/gitea/runner/internal/pkg/ver.version`. Affects anyone building with custom ldflags.
- **Kubernetes example resource names**: `act-runner` / `act-runner-vol` → `runner` / `runner-vol`. Users who copied the manifests verbatim will see resource churn on apply.
- **s6 service name**: `scripts/s6/act_runner/` → `scripts/s6/gitea-runner/` (image-internal; only matters for downstream image overrides).

Unchanged: YAML config field names, env vars (`GITEA_*`), CLI flags/subcommands, registration file format.
---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/850
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-committed-by: Lunny Xiao <xiaolunwen@gmail.com>

examples/README.md

@@ -1,4 +1,4 @@
-# Usage Examples for `act_runner`
+# Usage Examples for `gitea-runner`
 
 Welcome to our collection of usage and deployment examples specifically designed for Gitea setups. Whether you're a beginner or an experienced user, you'll find practical resources here that you can directly apply to enhance your Gitea experience. We encourage you to contribute your own insights and knowledge to make this collection even more comprehensive and valuable.