Modernize "Getting Started" Guide

[terrabitz]

This PR does a number of updates to modernize the recommendations:

• It adds a section about using automatic GitHub tokens to the "Getting Started" guide. This is generally easier and more secure than GitHub Apps. Over the years GitHub has changed what permissions are available to this token, and they now have a statuses option.
• It updates all the GitHub actions/* actions to their latest versions. This is done on the README, the "Getting Started" page, and the actual GHA workflows.
• It updates all Node.js references to version 24.x. This includes updates to the GHA actions, docs, and recipe folders. Node 18 is deprecated now, so we should switch to a current engine. If lhci has any issues with this version, I'm hoping they'll be surfaced through the CI checks

I did a basic test of these changes through a minimal PoC, which you can find here: terrabitz/lighthouse-ci-poc#1. In the PR, you can see that a status check was successfully created:

Implementation notes

• For the example code, I used a diff block since I personally like to see the highlighted changes when it comes to a smaller modification on a larger block. Downside is it makes it less copy-pastable. I'm open to suggestions

• Although the GitHub actions updates are technically semver breaking changes, they should be backwards-compatible; the only major difference is what Node engine version is used to precompile them. I personally have never had an issue during these upgrades.

• I opted to do all these changes together for ease, but please let me know if you want these changes broken up. I'd be happy to split them if that lowers the blast radius.

• This PR should cover Use latest GitHub Action for checkout #1111

• I have updated Node references in each CI provider's "Getting Started" section, but the only one I've personally tested is GHA. I don't have a good setup to test each of the other providers. I'm hoping because it's just image updates, it shouldn't break anything.

[nickspaargaren]

Node 18 has reached end of life and has been unsupported since April 2025. Let’s use Node 24 LTS.

Let's see if the CI passes 😉

[nickspaargaren]

Don't forget to update the node versions in the Dockerfiles as well;

• https://github.com/GoogleChrome/lighthouse-ci/blob/main/docs/recipes/docker-server/Dockerfile
• https://github.com/GoogleChrome/lighthouse-ci/blob/main/docs/recipes/docker-client/Dockerfile

[nickspaargaren]

Again, let's see if the CI passes 😉

[terrabitz]

@nickspaargaren even better! 🎉

• I updated all the refs to Node 24, including the Dockerfiles. I did a test build of both locally, though I wonder if it'd be worthwhile to add a CI check to make sure they're built correctly.
• I updated the Heroku recipe. I've never used Heroku before, but it didn't take too long to get setup to test that out too.
• I included an update for some of the other CI guides. Full disclosure, I haven't tested each of these myself. I'm hoping that since it's just image updates it should be fine:
  • Travis: Update to node_js v24
  • Circle CI: Update to cimg/node:24.13-browsers
  • GitLab CI: Update to cypress/browsers:24.13.0
  • Cloudbuild: Update to cypress/browsers:24.13.0 and node:24-alpine
    • (Not sure if we want a more specific cypress image, but I figured that's probably good for a general use case)
• Finally, in a couple of spots where we indicate "Node 16 and above", I replaced it with "Node 18 and above". I haven't done a full compatibility testing of each Node version to validate this claim, but it seems reasonable given my limited testing