[CI] Upgrade actions/checkout to the latest v6 version

[dnskr]

Why are the changes needed?

The changes are needed to keep CI dependencies up to date.

How was this patch tested?

CI

Was this patch authored or co-authored using generative AI tooling?

No

[cxzl25]

Keeping your actions up to date with Dependabot

https://docs.github.com/en/code-security/how-tos/secure-your-supply-chain/secure-your-dependencies/keeping-your-actions-up-to-date-with-dependabot

[pan3793]

thanks, merged to master

[dnskr]

Keeping your actions up to date with Dependabot

https://docs.github.com/en/code-security/how-tos/secure-your-supply-chain/secure-your-dependencies/keeping-your-actions-up-to-date-with-dependabot

@cxzl25 Thank you for the comment!

There is a GitHub Actions Policy document that requires projects to have automatic dependency management for GitHub Actions. I've created the issue Ensure GitHub Actions compliance with ASF Policy #7456 to address this and other related problems.
@pan3793 Could you please also have a look at it?