docs: add a supported versions matrix and pin minimum Kubernetes version

[myasnikovdaniil]

Context

Cozystack 1.4 bumps Flux to v2.8.0, which requires Kubernetes 1.33+ (see cozystack/cozystack#2602). Today the getting-started/requirements docs don't pin a minimum Kubernetes version, and there's no single page mapping a Cozystack release to its supported Kubernetes / Flux / Talos versions. Operators who don't read changelogs have no canonical place to check compatibility.

Goal

A supported versions matrix page, plus a pinned minimum Kubernetes version surfaced on the requirements page via the existing version-pin shortcode.

Plan

1. Add kubernetes_min (and flux_version) entries to the version-pin data, so they can be referenced like the existing talos_minor / cozystack_version pins.

2. Add a Supported versions page under docs/<version>/ (e.g. v1.4 and next) with a matrix:

   Cozystack	Kubernetes (min–max)	Flux	Talos	Notes

3. Update getting-started/requirements.md to state the minimum Kubernetes version using the pin, with a link to the matrix.

4. Backfill the matrix for currently supported releases (v1.3, v1.4).

Scope

Docs only. No code changes.

Reference

• feat(flux): upgrade to v2.8.0 + chart fixes for strict SSA & kstatus (folds #2612) cozystack#2602 — Flux v2.8 bump that surfaced the K8s 1.33 requirement.