Migrate RuntimeContext users to KernelRuntimeContext (#5270)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5270

`KernelRuntimeContext` is the stable name, and all code should use it. Update all uses of the old name in the //executorch tree.

This is all search-and-replace + lint, except for extension/kernel_util/README.md which I updated since the context is no longer a placeholder.

Patterns:
```
s/using RuntimeContext = torch::executor::RuntimeContext;/using executorch::runtime::KernelRuntimeContext;/
s/torch::executor::RuntimeContext\b/executorch::runtime::KernelRuntimeContext/g
s/\bRuntimeContext\b/KernelRuntimeContext/g
```

Reviewed By: shoumikhin

Differential Revision: D62479576

fbshipit-source-id: 237dcc8cf8ee2e9a246df21f526fb349fe48d064