Requirements.md: stale software versions, removed MySQL vars, no hardware sizing section

[somethingwithproof]

Problems

Stale software versions:

• PHP minimum listed as 5.4 — Cacti 1.2.31+ requires PHP 8.1
• MySQL minimum listed as 5.6 / MariaDB 5.5 — MySQL 5.6 and 5.7 are EOL; current minimum should be MySQL 8.0 or MariaDB 10.3+

Removed MySQL configuration variables (cause startup failure on MySQL 8):

• `innodb_additional_mem_pool_size` — removed MySQL 5.7.4
• `innodb_file_format = Barracuda` — removed MySQL 8.0 (in the paste-in config block at the bottom)
• `innodb_large_prefix = 1` — removed MySQL 8.0 (in the paste-in config block at the bottom)

No hardware sizing section:
There is no guidance on minimum or recommended server specifications (CPU cores, RAM, disk). New users have no starting point for provisioning. Spine.md mentions "4 GB RAM" for one data point but there is no sizing table covering small/medium/large deployments.

Proposed additions

Add a hardware sizing section covering:

• Small: < 500 devices / < 50k data sources — 2 CPU, 4GB RAM, 50GB SSD
• Medium: 500–2000 devices / 50k–500k data sources — 4 CPU, 16GB RAM, 200GB SSD
• Large: 2000+ devices / 500k+ data sources — 8+ CPU, 32GB+ RAM, 1TB+ NVMe, remote pollers recommended