[Android] build warning about deprecated APIs

## Bug Report

### Plugin(s)

@capacitor/filesystem

### Capacitor Version
```
Capacitor Doctor 💊

Latest Dependencies:

@capacitor/cli: 8.5.0
@capacitor/core: 8.5.0
@capacitor/android: 8.5.0
@capacitor/ios: 8.5.0

Installed Dependencies:

@capacitor/cli: 8.5.0
@capacitor/core: 8.5.0
@capacitor/android: 8.5.0
@capacitor/ios: 8.5.0

[success] iOS looking great! 👌
[success] Android looking great! 👌
```

### Platform(s)
Android

### Current Behavior

There are some build warnings:

```

[08:51:58]: ▸ > Task :capacitor-filesystem:compileReleaseKotlin
[08:51:58]: ▸ w: file:///home/runner/work/Site/Site/IsraelHiking.Web/node_modules/@capacitor/filesystem/android/src/main/kotlin/com/capacitorjs/plugins/filesystem/FilesystemPlugin.kt:335:31 'fun downloadFile(call: PluginCall): Unit' is deprecated. Use @capacitor/file-transfer plugin instead.
[08:51:58]: ▸ w: file:///home/runner/work/Site/Site/IsraelHiking.Web/node_modules/@capacitor/filesystem/android/src/main/kotlin/com/capacitorjs/plugins/filesystem/LegacyFilesystemImplementation.kt:66:29 Java type mismatch: inferred type is 'String?', but 'String' was expected.
[08:52:02]: ▸ Note: Recompile with -Xlint:unchecked for details.

```

### Expected Behavior
No build warnings

### Code Reproduction
I would assume any code that build Android with these plugins...

### Other Technical Details
Should probably be easy to solve using AI. I can open a PR if that would help move things along.

### Additional Context

Start with android/src/main/kotlin/com/capacitorjs/plugins/filesystem/FilesystemPlugin.kt around line 335 and LegacyFilesystemImplementation.kt around line 66, then run the Android release build that produced the warnings. Check the deprecated downloadFile call and nullable String warning against the current plugin APIs. Done means the filesystem plugin builds without these warnings.

Written by the indexing model from the issue text.