fix: clarify innodb_doublewrite trade-off and correct PHP minimum

innodb_doublewrite = OFF reduces crash safety on all storage types,
including SSD/NVMe; reword to make the durability trade-off explicit.
PHP minimum updated from 7.3 to 8.1 (required as of Cacti 1.2.31).

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>

Author: somethingwithproof

Requirements.md

@@ -17,10 +17,13 @@ Spine vs cmd.php, and whether remote pollers are used.
 
 - Use SSD or NVMe for the RRD file directory. RRDtool performs many small random
   writes; spinning disk causes polling backlogs on medium and larger installs.
-- The MySQL/MariaDB data directory benefits from SSD as well. Set
-  `innodb_doublewrite = OFF` only when the database is on SSD/NVMe, as it adds
-  write overhead that provides durability guarantees unnecessary on modern
-  hardware with battery-backed write caches.
+- The MySQL/MariaDB data directory benefits from SSD as well.
+  `innodb_doublewrite` protects against torn page writes during a crash;
+  disabling it reduces crash safety on any storage type, including SSD/NVMe.
+  Setting `innodb_doublewrite = OFF` is an advanced optimization; only
+  consider it when you accept the durability trade-off (e.g. strong
+  backup/replication strategy and tolerance for potential data loss on an
+  unclean shutdown).
 - Spine is CPU-bound. Each spine process spawns threads up to your configured
   maximum; allocate 1–2 spine processes per CPU core for best throughput.
 - Very large deployments (> 10,000 devices) require remote pollers deployed close to the devices
@@ -40,7 +43,7 @@ Cacti requires that the following software is installed on your system.
 
 - RRDtool 1.3 or greater, 1.5+ recommended
 
-- PHP 7.3 or greater, 8.0+ recommended
+- PHP 8.1 or greater
   - Required modules:
     - ctype, date, filter, gettext, gd, gmp, intl
     - hash, json, ldap, mbstring, openssl, pcre