Super Cache: use win_is_writable function instead of repeating that code in is_writeable_ACLSafe

### Impacted plugin

Super Cache

### Quick summary

On Windows, the filesystem security model is complicated and not supported entirely by PHP. To correctly know if a directory is writable, we must create a temporary file there.

In WordPress 2.8.0, the win_is_writable function was introduced to do this.

The function is_writeable_ACLSafe in Super Cache predates the win_is_writable() function, and both were taken from comments in the [is_writable](https://www.php.net/is_writable) PHP manual page.

As win_is_writable contains the same code, we should use that instead.

### Steps to reproduce

No issue, just want to use the WordPress native function instead.

### A clear and concise description of what you expected to happen.

N/A

### What actually happened

N/A

### Impact

One

### Available workarounds?

There is no user impact

### Platform (Simple and/or Atomic)

Self-hosted

### Logs or notes

_No response_