build(eslint-config-fluid): migrate @rushstack/no-new-null to @typescript-eslint/no-restricted-types

[tylerbutler]

Replace the Rushstack rule with the equivalent TypeScript ESLint rule
as recommended by the TypeScript ESLint migration guide.

Blocked by

• build(eslint-config-fluid): migrate from eslint-plugin-import to eslint-plugin-import-x #25661 (migrate import plugin)
• build(eslint-config-fluid): migrate to @eslint-community/eslint-plugin-eslint-comments #25662 (migrate eslint-comments)
• build(eslint-config-fluid): update TypeScript ESLint to v8 #25663 (update TypeScript ESLint)

Blocking

• build(eslint-config-fluid): update unicorn and jsdoc plugins #25665 (update minor dependencies)
• build(eslint-config-fluid): remove unused Rushstack plugins #25666 (remove Rushstack plugins)

[Copilot]

Pull Request Overview

This PR migrates from the Rushstack ESLint plugin rule @rushstack/no-new-null to the equivalent TypeScript ESLint rule @typescript-eslint/no-restricted-types as part of a broader effort to migrate away from Rushstack plugins. The migration maintains the same functionality of preventing the use of null in favor of undefined.

• Replaces @rushstack/no-new-null with @typescript-eslint/no-restricted-types configured to restrict null usage
• Updates both the recommended and minimal-deprecated ESLint configurations
• Maintains the same error/warning levels and provides equivalent functionality with auto-fix support

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
common/build/eslint-config-fluid/recommended.js	Migrates the rule from Rushstack to TypeScript ESLint with "error" severity
common/build/eslint-config-fluid/minimal-deprecated.js	Migrates the rule from Rushstack to TypeScript ESLint with "warn" severity and updates documentation