CI/CD: Upgrade Node.js 18.x to 20.x

[shivarm]

Upgrade Node.js 18.x to 20.x and deprecated github actions

• [x] There's a clear use-case for this code change, explained below
• [x] Commit message has a short title & references relevant issues
• [x] The build will pass (run yarn test:all and yarn lint)

We appreciate your contribution!

[markstos]

Why this is being done now? Only Node 18 is still officially supported:

https://ghost.org/docs/faq/node-versions/

I see no consideration here updated updating the documentation to say that Node 20 the new official version or any notes about the process of how the need to upgrade would be communicated to all users.

The Node 18 LTS will still receive security updates for another year.

[cmraible]

Hi @shivam-sharma7 , thanks for raising this! As @markstos mentioned, there is definitely more to this than just updating the tests in CI — we'll need to do some more thorough testing, update docs, etc. but thanks for bringing this up. Discussing with the team now, but I'll post an update here on v20.x when I have it 👍