Use Severity::Warning for miette warnings (#16146)

# Description


Changes miette warnings to use `Severity::Warning` instead of the
default `Severity::Error`. This shows them in a different color and with
a different icon:

<img width="650" height="266" alt="image"
src="https://github.com/user-attachments/assets/3ff0d3cf-ab1e-47f2-aff7-586ecea5a32a"
/>


# User-Facing Changes

* Warnings now look less like errors

crates/nu-protocol/src/errors/parse_warning.rs

@@ -7,6 +7,7 @@ use thiserror::Error;
 use super::ReportMode;
 
 #[derive(Clone, Debug, Error, Diagnostic, Serialize, Deserialize)]
+#[diagnostic(severity(Warning))]
 pub enum ParseWarning {
     #[error("{dep_type} deprecated.")]
     #[diagnostic(code(nu::parser::deprecated))]

crates/nu-protocol/src/errors/shell_error/mod.rs

@@ -1220,7 +1220,7 @@ This is an internal Nushell error, please file an issue https://github.com/nushe
     },
 
     #[error("{deprecation_type} deprecated.")]
-    #[diagnostic(code(nu::shell::deprecated))]
+    #[diagnostic(code(nu::shell::deprecated), severity(Warning))]
     DeprecationWarning {
         deprecation_type: &'static str,
         suggestion: String,