Everywhere: Rename {Deprecated => Byte}String

This commit un-deprecates DeprecatedString, and repurposes it as a byte
string.
As the null state has already been removed, there are no other
particularly hairy blockers in repurposing this type as a byte string
(what it _really_ is).

This commit is auto-generated:
  $ xs=$(ack -l \bDeprecatedString\b\|deprecated_string AK Userland \
    Meta Ports Ladybird Tests Kernel)
  $ perl -pie 's/\bDeprecatedString\b/ByteString/g;
    s/deprecated_string/byte_string/g' $xs
  $ clang-format --style=file -i \
    $(git diff --name-only | grep \.cpp\|\.h)
  $ gn format $(git ls-files '*.gn' '*.gni')

Meta/gn/secondary/AK/BUILD.gn

@@ -35,6 +35,7 @@ source_set("AK") {
     "BumpAllocator.h",
     "ByteBuffer.h",
     "ByteReader.h",
+    "ByteString.h",
     "CharacterTypes.h",
     "Checked.h",
     "CheckedFormatString.h",
@@ -50,7 +51,6 @@ source_set("AK") {
     "DefaultDelete.h",
     "Demangle.h",
     "DeprecatedFlyString.h",
-    "DeprecatedString.h",
     "Diagnostics.h",
     "DisjointChunks.h",
     "DistinctNumeric.h",
@@ -189,12 +189,12 @@ source_set("sources") {
   sources = [
     "Assertions.cpp",
     "Base64.cpp",
+    "ByteString.cpp",
     "CircularBuffer.cpp",
     "ConstrainedStream.cpp",
     "CountingStream.cpp",
     "DOSPackedTime.cpp",
     "DeprecatedFlyString.cpp",
-    "DeprecatedString.cpp",
     "Error.cpp",
     "FloatingPointStringConversions.cpp",
     "FlyString.cpp",