[5.x] Planned work for the next major

[cleptric]

This is a rough outline about the changes we're planning to make in the next major version of the PHP SDK.

• [x] Remove remaining Metrics code
• [x] #1897
• [x] #1896
• [ ] #1898
• [ ] #1899
• [ ] Remove the Hub and implement the new Global, Isolation and Current Scope concept
• [ ] Deprecate the Transactions API and implement a Span First API
• [ ] New propagators
• [x] #1921
• [ ] Better Pii controls, instead of send_default_pii
• [ ] Remove nullable clients on the hub
• [ ] Make sure the SDK is fully compatible with async PHP runtimes like FrankenPHP
• [ ] Be way more defensive with internal breakage
• [ ] No more ignored PHPStan errors

[cleptric]

~~We might also consider bumping the PHP version, based on current stats for 4.x.~~ We continue to support PHP 7.2.

https://packagist.org/packages/sentry/sentry/php-stats#4