AK+Everywhere: Rename String to DeprecatedString

We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)

AK/Utf8View.h

@@ -7,7 +7,7 @@
 
 #pragma once
 
-#include <AK/String.h>
+#include <AK/DeprecatedString.h>
 #include <AK/StringView.h>
 #include <AK/Types.h>
 
@@ -60,7 +60,7 @@ public:
 
     Utf8View() = default;
 
-    explicit Utf8View(String& string)
+    explicit Utf8View(DeprecatedString& string)
         : m_string(string.view())
     {
     }
@@ -72,7 +72,7 @@ public:
 
     ~Utf8View() = default;
 
-    explicit Utf8View(String&&) = delete;
+    explicit Utf8View(DeprecatedString&&) = delete;
 
     StringView as_string() const { return m_string; }