chore: fix legacy links and latest

[heitortsergent]

What?

• Fix a couple of links that were still pointing to k6.io/docs.
• Replace Grafana docs links that were using latest with <K6_VERSION>.

Checklist

Please fill in this template:

• 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 make docs command locally and verified that the changes look good.

If updating the documentation for the most recent release of k6:

• 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.