README.md: Update outdated references for WordPress 6.9/7.0

[itsgajendraSingh]

Extends #150. Related: #101.

Several sections of the README are outdated after the Abilities API merged into WordPress 6.9 core (Dec 2025), the WordPress/abilities-api repo was archived (Feb 2026), and the MCP Adapter shipped in WordPress 7.0 core (April 2026).

What needs updating

1. Abilities API still listed as a separate required dependency — repo was archived Feb 2026
2. composer require command includes wordpress/abilities-api (archived package)
3. No WordPress minimum version listed — only PHP >= 7.4
4. .wp-env.json example includes WordPress/abilities-api in plugins array
5. Basic Usage says all abilities are "automatically available" — missing meta.mcp.public requirement
6. Code example missing meta.mcp.public flag — ability won't appear on default server without it
7. Abilities API docs link points to archived GitHub repo instead of core dev notes
8. STDIO transport labeled "Testing Only" — it's the primary local dev transport for Claude/Cursor/VS Code
9. No explanation of what the @automattic/mcp-wordpress-remote proxy does