CI: Go 1.26 lint fails on deprecated io/ioutil usage

### Description

The `golangci-lint` job for the v0.7.5 tag fails when the workflow resolves `go-version: stable` to Go 1.26.5. Staticcheck reports SA1019 because `nocopy_readwriter_test.go` still imports the deprecated `io/ioutil` package.

The same commit's compatibility, Windows, license-header, and spelling jobs pass; this lint failure leaves the release workflow red even though `io.ReadAll` has provided the same operation since Go 1.16.

### Reproduction

1. Check out `d63d249a3ba0dc9675a85c9762242247c368e500` (v0.7.5).
2. Run the repository's `golangci-lint` job with Go stable (the failing run resolved Go 1.26.5 and golangci-lint v1.64.8).
3. Observe:

```text
nocopy_readwriter_test.go:22:2: SA1019: "io/ioutil" has been deprecated since Go 1.19
```

Failing job: https://github.com/cloudwego/netpoll/actions/runs/31067912905/job/92509503706

### Expected behavior

The repository's lint job should pass on its configured stable Go toolchain.

### Suggested fix

Replace `ioutil.ReadAll` with `io.ReadAll` and remove the deprecated import. This is test-only and does not change runtime behavior or supported APIs.

### Environment

- Repository commit: `d63d249a3ba0dc9675a85c9762242247c368e500`
- Release: v0.7.5
- CI toolchain shown in the failing job: Go 1.26.5, golangci-lint v1.64.8, Ubuntu 24.04

Open nocopy_readwriter_test.go and inspect the deprecated io/ioutil import and ReadAll usage described in the lint output. Replace that test-only usage with the current equivalent, remove the deprecated import, and rerun the repository's golangci-lint job with the stable Go toolchain; done means the lint job passes.

Written by the indexing model from the issue text.