Update part9c.md to use up-to-date information recommended by official docs

[sheikhameen]

Fixes issue #3747

Instead of installing these:

npm install express
npm install --save-dev eslint @types/express @typescript-eslint/eslint-plugin @typescript-eslint/parser

Install these:

npm install express
npm install --save-dev eslint @eslint/js typescript-eslint @stylistic/eslint-plugin @types/express @types/eslint__js

• eslint, @eslint/js, typescript-eslint - as recommended by eslint docs.
• @types/express - for express typing.
• @types/eslint__js - for typing @eslint/js
• @stylistic/eslint-plugin - for semi rule. (the "@typescript-eslint/semi" is deprecated and moved to ESLint Stylistic as mentioned here)

Updated to use the latest flat config file eslint.config.mjs instead of the deprecated eslintrc format.

[mluukkai]

fixed by another commit