Replace the 'k6 cloud' references with the new subcommands

[joanlopez]

What?

It replaces occurrences of k6 cloud (only the command, not the service name) with k6 cloud run, and k6 login cloud with k6 cloud login, which are the new subcommands that are (will be) part of v0.53 release.

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.

• I have made my changes in the docs/sources/next folder of the documentation.
• I have reflected my changes in the docs/sources/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

• I have made my changes in the docs/sources/next folder of the documentation.

Related PR(s)/Issue(s)

[heitortsergent]

Thank you for making these changes @joanlopez! 🙇