Preparation for v3.0.0

[acelaya]

• Remove deprecated stuff. #1275
• Remove default GeoLite2 license key and revoke it. Do not try to geolocate visits when no license key is provided.
• Disable default GeoLite2 license key.
• Drop support for PHP 7.4 (use some of the cool PHP 8 features). This will actually happen for version 2.8 Drop support for PHP 7.4 #1046
• Do some load tests and improve performance. #1276
• Reduce docker image size. #1277
• Remove URL validation config option. Only rely in the validateUrl param. If it's sent with value true, the URL is validated, otherwise it's not.
• Enable composer platform-checks: This will check all required extensions are installed when running shlink, throwing an error otherwise. Not possible, as there are features that require specific platform deps, and forcing to install those even if you are not using the features, will cause friction.
• Disable "delete threshold" if the DELETE_SHORT_URL_THRESHOLD env var is not provided. #1283
• Make USE_HTTPS IS_HTTPS_ENABLED default to true.
• Re-consider domain falling back behavior. It was done the way it is for BC. Might be the moment to change it. This can't be changed, as it would make it hard to update to v3.0 from older versions but retaining links behavior.
• Order short URLs from newest to oldest if no ordering is provided. #1300
• Update to latest openswoole and base docker images #1299
• Check pending TODOs and FIXMEs.
• Drop support (soft) for regular swoole in favor of openswoole #1347
• Review README for outdated info
• Ensure only known env vars can be loaded from generated_config.php