LibCrypto: Don't return reference to stack frame in PBKDF

A reference to the current stack frame becomes invalid after returning,
so returning Bytes is pointless.

I don't understand why this wasn't discovered earlier, but it caused
some CI problems for me, so I fixed it.

Don't take this as encouragement to break master! :^)

Userland/Libraries/LibCrypto/Hash/PBKDF2.h

@@ -15,7 +15,7 @@ namespace Crypto::Hash {
 class PBKDF2 {
 public:
     template<typename PRF>
-    static ErrorOr<Bytes> derive_key(ReadonlyBytes password, ReadonlyBytes salt, u32 iterations, u32 key_length_bytes)
+    static ErrorOr<ByteBuffer> derive_key(ReadonlyBytes password, ReadonlyBytes salt, u32 iterations, u32 key_length_bytes)
     requires requires(PRF t) {
                  t.digest_size();
              }