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>

.gitea/workflows/test.yml

@@ -12,8 +12,8 @@ jobs:
       - uses: actions/setup-go@v6
         with:
           go-version-file: 'go.mod'
-      - name: vet checks
-        run: make vet
+      - name: lint
+        run: make lint
       - name: build
         run: make build
       - name: test