feat(vision): make default api version configurable

[sjelfull]

Description

This increases the default API version in Vision to v2021-10-21, and makes it configurable via config file config/@sanity/vision.json.

What to review

• That the config convention is the one we prefer (config: vs part: import)
• That the readme update on how to configure Vision reads okay

Notes for release

• Set default API version in Vision to v2021-10-21
• Made default API version in Vision configurable

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perf-studio – ./

🔍 Inspect: https://vercel.com/sanity-io/perf-studio/2MWWFk6xKSBjT677xmfLCYxNjuU9
✅ Preview: https://perf-studio-git-feature-sc-13580set-different-default-gr-939b2a.sanity.build

test-studio – ./

🔍 Inspect: https://vercel.com/sanity-io/test-studio/h8BokCi5nvZc38wenyUmypAtjL4K
✅ Preview: https://test-studio-git-feature-sc-13580set-different-default-gr-79e3cb.sanity.build

studio-workshop – ./dev/workshop

🔍 Inspect: https://vercel.com/sanity-io/studio-workshop/5LJZNhQQNDYVNVNY9SzuieEXmYGs
✅ Preview: https://studio-workshop-git-feature-sc-13580set-different-defaul-0d90b5.sanity.build

[bjoerge]

Nice ✨