[PHP8.1] Document minimum version for PHP Memcached extention: 3.2.0

Problem/Motivation

The PHP Memcached extension 3.2.0 contains an important bug fix that causes float values to be stored as negative values: https://github.com/php-memcached-dev/php-memcached/issues/500

When Drupal runs 'Flush all caches', the Memcache module implements this as a 'last bin deletion time' as a float. As a result, Drupal never flushes memcache when running an outdated version of the PHP Memcached extension.

Proposed resolution

Document minimum version for PHP Memcached extention: 3.2.0

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.