Rename the v2 gsutil download section

What does this MR do and why?

Renames "Using the gsutil tool to download the package metadata exports" in doc/topics/offline/quick_start_guide.md to "Download v2 package metadata with gsutil". The old title says how you download rather than what you get, and the section covers v2 only. Renaming changes the anchor, so the inbound link from the troubleshooting section is updated too.

Requested by @nilieskou in note_3780137769.

Relates to

• #627684 (closed)
• Stacked on !257844 (merged). !253578 (merged) targets this branch.