ci: use corepack to automatically handle pnpm version & use GitHub composite actions

[MuhammadM1998]

Currently the pnpm version is hardcoded in workflows file and needs to be updated manually like this 4fc463a (#531) otherwise CI fails.

This PR does the following

• Enables corepack which automatically uses the specified version by packageManager in package.json.
• Moves the shared setup (Installing node & enabling corepack) into a GitHub Composite Action which is basically a reusable workflow that saves us duplication of steps across the workflows.
• Bumps actions/checkout to v4 for consistency as both v2 and v3 were used in different workflows.
• Replaces npx with pnpm dlx in changelogithub script for consistency as we're using pnpm

[sadeghbarati]

@MuhammadM1998 🙌