docs: update README Firefox not found example

[MikeMcC399]

Issue

README > Firefox not found refers to outdated GitHub JavaScript action versions, which use the deprecated / end-of-life node16 version:

name: E2E in custom container
on: push
jobs:
  cypress-run:
    runs-on: ubuntu-22.04
    container:
     image: cypress/browsers:node18.12.0-chrome106-ff106
     options: --user 1001
    steps:
      - uses: actions/checkout@v3
      - uses: cypress-io/github-action@v5
        with:
          browser: firefox

• actions/checkout@v3 using node16
• [cypress-io/github-action@v5]()

Change

Update the example in README > Firefox not found to use latest and supported versions:

• actions/checkout@v4
• cypress-io/github-action@v6

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[MikeMcC399]

Preview

https://github.com/MikeMcC399/cypress-docker-images/blob/ff-not-found-example/README.md#firefox-not-found