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>

.goreleaser.yaml

@@ -1,5 +1,7 @@
 version: 2
 
+project_name: gitea-runner
+
 before:
   hooks:
     - go mod tidy
@@ -63,7 +65,7 @@ builds:
   flags:
   - -trimpath
   ldflags:
-  - -s -w -X gitea.com/gitea/act_runner/internal/pkg/ver.version={{ .Summary }}
+  - -s -w -X gitea.com/gitea/runner/internal/pkg/ver.version={{ .Summary }}
   binary: >-
     {{ .ProjectName }}-
     {{- .Version }}-
@@ -86,7 +88,7 @@ blobs:
     provider: s3
     bucket: "{{ .Env.S3_BUCKET }}"
     region: "{{ .Env.S3_REGION }}"
-    directory: "act_runner/{{.Version}}"
+    directory: "gitea-runner/{{.Version}}"
     extra_files:
       - glob: ./**.xz
       - glob: ./**.sha256