Correct state of "Local File Content Sniffing" flag

https://bugs.webkit.org/show_bug.cgi?id=255673
<rdar://problem/108277544>

Reviewed by Patrick Angle.

The "Local File Content Sniffing" flag is an internal debugging flag, and not
a feature flag users or developers need to interact with.

Stable is not the right status for this flag; it should be `internal` instead.

* Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml:

Canonical link: https://commits.webkit.org/263150@main

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -3760,7 +3760,7 @@ LoadsSiteIconsIgnoringImageLoadingPreference:
 
 LocalFileContentSniffingEnabled:
   type: bool
-  status: stable
+  status: internal
   category: networking
   humanReadableName: "Local File Content Sniffing"
   humanReadableDescription: "Enable Local File Content Sniffing"