Remove references to math/rand package's Read function.

[copybara-service[bot]]

Remove references to math/rand package's Read function.

The helper function is deprecated. The package gvisor.dev/gvisor/pkg/rand depends on crypto/rand which performs worse thatn math/rand, the changes are fine since they are not at any gVisor's hot path.

The ultimate goal is to migrate math/rand to math/rand/v2.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/google/gvisor/pull/11030 from milantracy:rand 7cc17225e694741720e3da7fb87164b6d6e1b8a3