updates let-env signature to remove required params (#9917)

# Description

This PR changes the signature of the deprecated command `let-env` so
that it does not mislead people when invoking it without parameters.

### Before
```nushell
> let-env
Error: nu::parser::missing_positional

  × Missing required positional argument.
   ╭─[entry #2:1:1]
 1 │ let-env
   ╰────
  help: Usage: let-env <var_name> = <initial_value>
```

### After
```nushell
❯ let-env
Error: nu:🐚:deprecated_command

  × Deprecated command let-env
   ╭─[entry #1:1:1]
 1 │ let-env
   · ───┬───
   ·    ╰── 'let-env' is deprecated. Please use '$env.<environment variable> = ...' instead.
   ╰────
```

# User-Facing Changes
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# Tests + Formatting
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crates/nu-command/src/deprecated/let_env.rs

@@ -14,8 +14,8 @@ impl Command for LetEnvDeprecated {
         Signature::build(self.name())
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .allow_variants_without_examples(true)
-            .required("var_name", SyntaxShape::String, "variable name")
-            .required(
+            .optional("var_name", SyntaxShape::String, "variable name")
+            .optional(
                 "initial_value",
                 SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
                 "equals sign followed by value",