[core] Pin Node version on GitHub Actions to 18.18

[michaldudak]

ts-node doesn't work with Node 18.19 (see TypeStrong/ts-node#2094), causing issues with the validate-declarations script.
Pinning Node version on GitHub Actions for now. If ts-node is broken for a longer time, we'll think of a better solution.

Fixes #40173

[ZeeshanTamboli]

The script was updated in #39947. Should we change it back?

[lhilgert9]

The script was updated in #39947. Should we change it back?

@ZeeshanTamboli I hadn't seen this change, but in principle it was the second option I suggested in #40173. So either undo the change to the script and change the Node.js version or keep the script and leave Node.js at 18.19.

[Janpot]

Just copying the notes from decision making in our code infra meeting:

• let’s revert node.js version for now
• let's revert changes to the ts-node command
• let's update renovate.json to isolate node.js with a "description" field pointing to a github issue

[mui-bot]

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[michaldudak]

It turns out that #40080 is not the cause of this problem, as it updates Node on CircleCI, but the validate-declarations scripts runs on GitHub Actions. I updated this PR pinning the Node version for GH Actions and leaving the CircleCI version unpinned.