Set split-by doc category to "deprecated" (#14633)

# Description

#14019 deprecated the `split-by` command. This sets its doc-category to
"deprecated" so that it will display that way in the in-shell and online
help

# User-Facing Changes

`split-by` will now show as a deprecated command in Help. Will also be
reported using:

```nushell
help commands | where category == deprecated
```

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

crates/nu-command/src/filters/split_by.rs

@@ -14,7 +14,7 @@ impl Command for SplitBy {
         Signature::build("split-by")
             .input_output_types(vec![(Type::record(), Type::record())])
             .optional("splitter", SyntaxShape::Any, "The splitter value to use.")
-            .category(Category::Filters)
+            .category(Category::Deprecated)
     }
 
     fn description(&self) -> &str {