chore: update C++ server-side SDK from 3.10.1 to 3.12.0 and Redis Source from 2.1.19 to 2.3.1

[kinyoklion]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

Supersedes #123 (which only updated Redis Source to 2.2.2; both dependencies have newer releases now).

Describe the solution you've provided

Updates both C++ SDK dependencies to the latest stable versions:

• C++ Server-side SDK: 3.10.1 → 3.12.0
• C++ Server-side SDK Redis Source: 2.1.19 → 2.3.1

Files changed:

• .github/variables/cpp-sdk-versions.env — central version config used by CI
• examples/hello-haproxy/Dockerfile, examples/hello-nginx/Dockerfile, examples/hello-debian/Dockerfile — default CPP_SDK_VERSION ARG for local builds
• README.md — version table updated

How to test it

CI runs Docker builds for hello-haproxy, hello-nginx, and hello-debian examples against these versions.

Additional context

C++ SDK 3.12.0 adds C bindings for FDv2 data system. Redis Source 2.3.1 bumps internal C++ server SDK dependency to 3.12.0. No breaking changes or deprecated APIs.

Link to Devin session: https://app.devin.ai/sessions/719e31f6d5f64cb7bcfab0052762628e
Requested by: @kinyoklion

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Note

Low Risk
Version-pin and documentation updates only; integration risk is covered by existing CI Docker example builds.

Overview
Bumps the pinned LaunchDarkly C++ server-side SDK from 3.10.1 to 3.12.0 and the optional Redis Source from 2.1.19 to 2.3.1 across CI config, docs, and example builds.

.github/variables/cpp-sdk-versions.env is the source of truth for CI (via the cpp-versions composite action). The README minimum-version table and release links are updated to match. CPP_SDK_VERSION defaults in hello-debian, hello-haproxy, and hello-nginx Dockerfiles are raised so local/CI example images build against the new SDK tag or prebuilt artifact URL.

No Lua module or binding code changes—only dependency version alignment.

^{Reviewed by Cursor Bugbot for commit f5fb6fb. Bugbot is set up for automated code reviews on this repo. Configure here.}

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