API: don't rely on the version object for builds

[stsewd]

If a version is deleted, we still keep its builds around, so we can't rely on the version being present. Ref https://read-the-docs.sentry.io/issues/3961831199/?project=148442

[read-the-docs-community]

Documentation build overview

📚 dev | 🛠️ Build #29177332 | 📁 Comparing 0ec1f98 against latest (6d4243e)

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File	Status
contribute.html	📝 modified
install.html	📝 modified
search-integration.html	📝 modified
settings.html	📝 modified
style-guide.html	📝 modified
design/embed-api.html	📝 modified
design/theme-context.html	📝 modified

[read-the-docs-community]

Documentation build overview

📚 docs | 🛠️ Build #29177333 | 📁 Comparing 0ec1f98 against latest (6d4243e)

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build-customization.html	📝 modified
support.html	📝 modified
about/index.html	📝 modified
tutorial/index.html	📝 modified