Support OpenAPI 3.2.0 documents (standalone mode)

[yluom]

Describe the problem to be solved

The OpenAPI 3.2.0 specification was released in September 2025. When loading an OpenAPI
3.2.0 document, Redoc rejects it with: "Unsupported OpenAPI version: 3.2.0". This blocks any project that has upgraded its API
contract to OAS 3.2 from rendering documentation with Redoc.

Describe the solution you'd like

Redoc should parse and render OpenAPI 3.2.0 documents the same way it handles 3.0.x and 3.1.x today. Since 3.2 is a minor
increment over 3.1, the core rendering logic should require minimal changes — the main blocker appears to be the version
validation check.

At minimum:

• Accept openapi: 3.2.x in the version check
• Handle any new 3.2-specific constructs gracefully (degrade rather than error if not yet fully supported)

Describe alternatives you've considered

• Downgrading the spec to 3.1.0 — works but prevents using 3.2 features and forces contract divergence from the source of
  truth.
• Using Swagger UI — supports 3.2 but lacks the single-page rendering and customization that Redoc provides.
• Using Redocly CLI preview — the CLI has started adding 3.2 support, but redoc itself does not benefit from it yet.

Additional context

• Related: Support OpenAPI 3.2 documents #2746
• The redocly-cli has begun incremental 3.2 support (see referenced PRs), but redoc hasn't received these changes yet.
• OAS 3.2 is largely backwards-compatible with 3.1, so a pragmatic first step could be relaxing the version gate to 3.2.x
  while progressively adding full support.