👷 use latest major version for actions/checkout

[thomasmerz]

Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

Use latest major version of actions/checkout for GitHub Actions.

[dirkf]

What would be useful is to go through the CI here, and in the dependent actions repos here, and in the nightly repo (separate PRs, obvs) changing all uses to v4 **and setting progress: false in the with parameter list (see the action changelog and PR 1067 describing this). If you, or anyone, does do so, pls announce any new PRs from the other repos here.