SystemMonitor: Prefer File::read_until_eof over DeprecatedFile

Because MemoryStatsWidget::refresh is run nearly directly by the
EventLoop, the only real alternative to crashing would be to cancel the
update, and keep the old data. That doesn't sound sufficiently better to
warrant being implemented.

Userland/Applications/SystemMonitor/MemoryStatsWidget.cpp

@@ -9,7 +9,6 @@
 #include "GraphWidget.h"
 #include <AK/JsonObject.h>
 #include <AK/NumberFormat.h>
-#include <LibCore/DeprecatedFile.h>
 #include <LibCore/Object.h>
 #include <LibGUI/BoxLayout.h>
 #include <LibGUI/Label.h>
@@ -102,11 +101,9 @@ static inline u64 page_count_to_bytes(size_t count)
 
 void MemoryStatsWidget::refresh()
 {
-    auto proc_memstat = Core::DeprecatedFile::construct("/sys/kernel/memstat");
-    if (!proc_memstat->open(Core::OpenMode::ReadOnly))
-        VERIFY_NOT_REACHED();
+    auto proc_memstat = Core::File::open("/sys/kernel/memstat"sv, Core::File::OpenMode::Read).release_value_but_fixme_should_propagate_errors();
 
-    auto file_contents = proc_memstat->read_all();
+    auto file_contents = proc_memstat->read_until_eof().release_value_but_fixme_should_propagate_errors();
     auto json_result = JsonValue::from_string(file_contents).release_value_but_fixme_should_propagate_errors();
     auto const& json = json_result.as_object();