composer: Switch to latest dependencies

[jtojnar]

We now target PHP 8.1, compatibility with older PHP versions will be provided using code transformation with Rector.

• Remove config.platform.php from composer since it forces versions to be installed.
• Dice is now immutable, we need to overwrite the variable. Also change the container to inject the proper Dice instance.
• monolog\Logger expects a string argument so we need to set up DI to provide it one in unit tests.
• monolog deprecated the warn method.
• Graby now depends on tidy

[jtojnar]

This was superseded by #1392