centipede::BlobSequence uses deprecated is_pod

https://github.com/google/fuzztest/blob/main/centipede/shared_memory_blob_sequence.h#L72

```
// Writes `tag`/`value` as a blob. `T` should be a POD.
// Returns true on success.
template
bool Write(Blob::SizeAndTagT tag, T value) {
static_assert(std::is_pod_v, "T must be a POD");
return Write(
{tag, sizeof(value), reinterpret_cast(&value)});
}
```

is_pod is deprecated in C++20 and was marked as such in libc++ in https://github.com/llvm/llvm-project/pull/129471

This probably wants to use std::is_standard_layout instead.