Prepare 2.0: PHP 8.2+, PHPUnit 11.5+, PHPStan 2 at max level

[mnapoli]

Cleanup for the 2.0 major, after #2.

• PHP 8.2+ (was 8.0+). Monolog ^2.4|^3 is unchanged: apps on Monolog 2 (e.g. Symfony 5.4) keep working.
• Dev dependencies: PHPUnit ^11.5|^12|^13 (11.5 is the first to require PHP 8.2), PHPStan ^2.
• PHPStan at level max (was 5). Monolog 2 records (arrays) and Monolog 3 records (LogRecord) are now read in one place, fields(), with a typed shape, instead of $record['level_name'] ?? $record['level']['name'] everywhere. formatBatch() declared its parameter as a single record rather than an array of records.
• CI: PHP 8.2 to 8.5, --prefer-lowest on 8.2 (which tests Monolog 2.4 and PHPUnit 11.5), phpcs now runs (the "Coding standards" job only ran PHPStan, which is how an unused import landed on main), actions/checkout@v7 and actions/cache@v6, no more deprecated --no-suggest, cache key on composer.json since there is no lock file.
• phpstan.neon is excluded from the package archive.
• README: the Laravel and Symfony bridges set this formatter by default; the Symfony example uses the bref.cloudwatch_formatter service registered by BrefBundle (the class name only works if the app registers it as a service) and logs to php://stderr rather than to a file.

Behavior: unchanged, except that closed resources in the context are now normalized by Monolog ([unknown(resource (closed))]) instead of being passed as-is to json_encode(). Level names were already uppercase in Monolog 2 and 3, so strtoupper() is gone.

Tested locally with the latest dependencies (Monolog 3.12, PHPUnit 13) and the lowest ones (Monolog 2.4, PHPUnit 11.5).