Document RDI API v1 deprecation and migration

[ZdravkoDonev-redis]

Note

Low Risk
Documentation-only changes with no runtime or API behavior impact.

Overview
Documents RDI API v1 deprecation as of RDI 1.19.0 and steers integrators toward API v2.

Adds a dedicated API migration page with v1→v2 endpoint mapping, notes on v1-only endpoints (login, templates, etc.), and guidance to replace action-ID polling with pipeline-scoped status polling (including curl examples). The Reference index, 1.19.0 release notes, and OpenAPI info.description (shown in Redoc) now surface the same deprecation message and link to the migration guide; api-reference.md documents that Redoc ignores the markdown body so the notice lives in the spec.

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[github-actions]

Staging links:
https://redis.io/docs/staging/RDSC-5718-rdi-api-v1-deprecation/integrate/redis-data-integration/reference/
https://redis.io/docs/staging/RDSC-5718-rdi-api-v1-deprecation/integrate/redis-data-integration/reference/api-migration/
https://redis.io/docs/staging/RDSC-5718-rdi-api-v1-deprecation/integrate/redis-data-integration/reference/api-reference/
https://redis.io/docs/staging/RDSC-5718-rdi-api-v1-deprecation/integrate/redis-data-integration/release-notes/rdi-1-19-0/

[stoyanr]

The bot made a mess of this one :-(

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

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[andy-stark-redis]

@ZdravkoDonev-redis The note you originally added to the API reference Markdown won't be visible (this page uses a special layout to render the reference). I've added a change to openapi.json to render the note correctly and also made a corresponding change in a PR on the RDI repo.

[andy-stark-redis]

I'd recommend the suggested changes before merging (I can commit these myself if you're happy with them), but they're easy enough to add - I'll approve to avoid delays.