Allow psr/log 3 next to psr/log 2 #599

Summary

Allow psr/log 3 next to 2 (^2.0 || ^3.0), so applications that already use psr/log 3, for example through Monolog 3, can install Quantum. Part of the PHP 8.1 update (#595).

Why

composer.json requires psr/log ^2.0. Composer reports 3.0.2 as installable on PHP 8.1 but blocked by that constraint.

Widening the constraint is not enough on its own: symfony/console 5.4, which we require, declares a conflict with psr/log 3 or newer, so Composer cannot install psr/log 3 until the console moves to 6.4 (#596). This ticket therefore comes after #596. Quantum\Logger\Logger implements Psr\Log\LoggerInterface, and PHPMailer accepts any PSR-3 logger.

Current state

• psr/log is used in 2 files under src: Logger (implements LoggerInterface) and ExceptionSeverityResolver (uses LogLevel).
• Logger already declares : void on all level methods and log().

Proposed change

• composer.json: "psr/log": "^2.0 || ^3.0".
• Run the Logger tests (and the Mailer tests that use the logger) against both versions, for example with composer update psr/log --with psr/log:^3.0.
• Mention the wider constraint in the changelog.

Depends on

• Move symfony/console, var-dumper and var-exporter to the 6.4 LTS line #596, which moves symfony/console off 5.4.

Out of scope

• psr/simple-cache 3 (its interface adds native types to every cache adapter, which is a breaking change for custom adapters).

Acceptance criteria

• the Logger and Mailer tests pass with psr/log 2 and with psr/log 3
• composer cs:check, composer phpstan and composer rector:check pass with both

Notes

Relevant code: composer.json, src/Logger/Logger.php.