Use the new `std.Io.Writer`

Removes references to the deprecated `std.io.Writer` from `std.fs.File.Writer`. There's a [writeup here](https://www.openmymind.net/Zigs-New-Writer/). [Zig 0.15](https://github.com/ziglang/zig/milestone/28) is soon to be released (although they recently removed the due date of 2025-08-12). I still feel that preparation for this change is valid.