chore(lint): add golangci-lint v2 and fix all lint issues (#803)

## Summary
- Replace old `.golangci.yml` (v1 format) with v2 format, aligned with gitea's lint config
- Add `lint-go`, `lint-go-fix`, and `lint` Makefile targets using golangci-lint v2.10.1
- Replace `make vet` with `make lint` in CI workflow (lint includes vet)
- Fix all 35 lint issues: modernize (maps.Copy, range over int, any), perfsprint (errors.New), unparam (remove unused parameters), revive (var naming), staticcheck, forbidigo exclusion for cmd/
- Make `security-check` non-fatal (apply https://github.com/go-gitea/gitea/pull/36681)
- Remove dead gocritic exclusion rules (commentFormatting, exitAfterDefer)
- Remove dead linter exclusions and disabled checks (singleCaseSwitch, ST1003, QF1001, QF1006, QF1008, testifylint go-require/require-error, test file exclusions for dupl/errcheck/staticcheck/unparam)

## Test plan
- [x] `golangci-lint run` passes
- [x] `go build ./...` passes
- [x] `go test ./...` passes

---------

Co-authored-by: ChristopherHX <christopher.homberger@web.de>
Co-authored-by: Christopher Homberger <christopher.homberger@web.de>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/803
Reviewed-by: ChristopherHX <christopherhx@noreply.gitea.com>

Makefile

@@ -20,6 +20,7 @@ DOCKER_TAG ?= nightly
 DOCKER_REF := $(DOCKER_IMAGE):$(DOCKER_TAG)
 DOCKER_ROOTLESS_REF := $(DOCKER_IMAGE):$(DOCKER_TAG)-dind-rootless
 
+GOLANGCI_LINT_PACKAGE ?= github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.10.1
 GOVULNCHECK_PACKAGE ?= golang.org/x/vuln/cmd/govulncheck@v1
 
 ifneq ($(shell uname), Darwin)
@@ -107,9 +108,20 @@ fmt-check:
 deps-tools: ## install tool dependencies
 	$(GO) install $(GOVULNCHECK_PACKAGE)
 
+.PHONY: lint
+lint: lint-go vet
+
+.PHONY: lint-go
+lint-go: ## lint go files
+	$(GO) run $(GOLANGCI_LINT_PACKAGE) run
+
+.PHONY: lint-go-fix
+lint-go-fix: ## lint go files and fix issues
+	$(GO) run $(GOLANGCI_LINT_PACKAGE) run --fix
+
 .PHONY: security-check
 security-check: deps-tools
-	GOEXPERIMENT= $(GO) run $(GOVULNCHECK_PACKAGE) -show color ./...
+	GOEXPERIMENT= $(GO) run $(GOVULNCHECK_PACKAGE) -show color ./... || true
 
 .PHONY: tidy
 tidy: