Bump minimum PHP and dependency versions

[jasonvarga]

This PR raises the minimum laravel/prompts version to 0.3.19, which is the version that introduced callout(). It also covers task() with keepSummary (added in 0.3.17), which is used since #110. #114 will use callout().

Since #111 recommends installing the CLI with cpx, bumping dependencies should be fine.

• Requires laravel/prompts ^0.3.19
• Removes the keepSummary fallback check for older Prompts versions
• Raises composer-runtime-api to ^2.2 and symfony/console to ^6.2|^7.0, to match what Prompts 0.3.19 already requires
• Drops symfony/process 4.x and 5.x to line up with console
• Requires PHP 8.2
• Adds a README troubleshooting section that recommends switching to cpx if a global install runs into Composer version conflicts