Prepare 3.0: PHP 8.2+, PHPUnit 11.5+, PHPStan 2 at max level

[mnapoli]

Cleanups for the 3.0 major release, after #12.

Requirements

• PHP 8.2+ (was 8.0). psr/log stays ^1.1 | ^2.0 | ^3.0.
• Dev: PHPUnit ^11.5|^12|^13 (11.5 is the first series on PHP 8.2), PHPStan ^2.

CI

• PHP 8.2 to 8.5, with --prefer-lowest on 8.2.
• actions/checkout@v7, actions/cache@v6, coding standards on PHP 8.5.
• Removed Composer's deprecated --no-suggest.

Code (no change to the public API)

• PHPStan at level max (was 5), for src and tests, without baseline or ignores.
• PHP 8 syntax: constructor property promotion, mixed, str_contains(), $object::class, Stringable.
• The stream and its URL are one property, opened lazily as before.

Behavior changes

• An unsupported log level (e.g. $logger->log('warn', …)) now throws Psr\Log\InvalidArgumentException, as PSR-3 requires. It used to trigger a PHP warning ("Undefined array key") and drop the message.
• A Stringable message (allowed by psr/log 2 and 3) is now logged. It used to throw a TypeError with strict_types.
• A value that cannot be encoded to JSON now throws a JsonException instead of a TypeError.

Other

• .gitattributes: .phpcs.xml.dist (the actual file name) and phpstan.neon are excluded from the package.
• README: documentation links point to the current URL (without .html).