Prepare 3.8.0

[smorimoto]

Summary

Bump version to 3.8.0 for the next release.

Changes since v3.7.2

Features

• Allow action-managed caching to be disabled and reuse existing local switches when caching is disabled (Allow action-managed caching to be disabled #1125)

Improvements

• Validate opam output, repository mappings, and runner environment values with Zod (Validate external inputs with Zod #1128)
• Update CI, documentation, and examples to OCaml 5.5 (Bump CI and docs to OCaml 5.5 #1126)

Internal

• Add Knip dead-code checks and clean up dependency declarations
• Bump Vite Plus from 0.2.9 to 0.3.0
• Group updates to GitHub Actions maintained by the OCaml organisation (Group OCaml action updates in Dependabot #1127)

Testing

• yarn install --immutable
• yarn lint
• yarn knip
• yarn typecheck
• yarn build