AK: Rename Time to Duration

That's what this class really is; in fact that's what the first line of
the comment says it is.

This commit does not rename the main files, since those will contain
other time-related classes in a little bit.

Kernel/FileSystem/ProcFS/Inode.h

@@ -54,7 +54,7 @@ private:
 
     // ^Inode (Silent ignore handling)
     virtual ErrorOr<void> flush_metadata() override { return {}; }
-    virtual ErrorOr<void> update_timestamps(Optional<Time>, Optional<Time>, Optional<Time>) override { return {}; }
+    virtual ErrorOr<void> update_timestamps(Optional<Duration>, Optional<Duration>, Optional<Duration>) override { return {}; }
 
     // ^Inode
     virtual ErrorOr<void> attach(OpenFileDescription& description) override;