Update using_kaniko.md

What does this MR do?

Update documentation to reflect proper kaniko usage, especially as it pertains to token renewal.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/50750

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides