Support PHP 7.4 to 8.5

[TomasVotruba]

Makes jack installable and runnable natively across PHP 7.4 to 8.5, so it can be required directly in projects on older PHP. Inspired by TomasVotruba/class-leak#91, now that entropy/entropy runs on 7.4.

Source backport (7.4 syntax, still valid up to 8.5)

• readonly removed, constructor property promotion expanded to explicit typed properties
• typed class constants (const string) moved to plain constants
• new DateTimeImmutable(...)->diff(...) (PHP 8.4 chaining) split into two statements
• named argument timeout: 120 on Process::fromShellCommandline() moved to positional
• symfony/polyfill-php80 added for str_contains / str_starts_with

Distribution

• composer.json php constraint lowered from >=8.4 to >=7.4
• symfony/process widened to ^5.4 || ^6.4 || ^7.0 || ^8.1, so 7.4 resolves against Symfony 5.4 LTS
• the Rector downgrade build pipeline is dropped, since the source now runs on 7.4 directly: removed downgraded_release.yaml, bare_run.yaml, build/, scoper.php, prefix-code.sh, full-tool-build.sh

Tests and CI

• test suite moved to PHPUnit 9.6, the version that runs identically on 7.4 through 8.5; data providers switched from attributes to annotations
• new Tests workflow: matrix 7.4, 8.2, 8.5 running PHPUnit and a CLI smoke run, plus a parallel-lint job on 7.4 and 8.5
• analysis-only tools (PHPStan, ECS, Rector, symplify, swiss-knife, class-leak, dependency-analyser, tracy) are removed before install in the low-PHP jobs, as they require PHP 8.2+ and are not needed to run the tests
• Rector config targets php74 so it keeps the source at 7.4 syntax

Existing Code Analysis workflow (PHPStan, ECS, Rector) stays on PHP 8.4.

PHPUnit, PHPStan, ECS, Rector and parallel-lint all pass locally.