[Rust Server] Update rust-server cargo-config to generate .cargo/config.toml (#17876)

This is recommended by Rust 1.39:

<https://doc.rust-lang.org/cargo/reference/config.html>

Author: azriel-healthpoint

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/RustServerCodegen.java

@@ -222,7 +222,7 @@ public RustServerCodegen() {
          */
         supportingFiles.add(new SupportingFile("openapi.mustache", "api", "openapi.yaml"));
         supportingFiles.add(new SupportingFile("Cargo.mustache", "", "Cargo.toml"));
-        supportingFiles.add(new SupportingFile("cargo-config", ".cargo", "config"));
+        supportingFiles.add(new SupportingFile("cargo-config", ".cargo", "config.toml"));
         supportingFiles.add(new SupportingFile("gitignore", "", ".gitignore"));
         supportingFiles.add(new SupportingFile("lib.mustache", "src", "lib.rs"));
         supportingFiles.add(new SupportingFile("context.mustache", "src", "context.rs"));