[RFC] Bump minimal PHP version to 7.4

[malarzm]

https://packagist.org/packages/doctrine/mongodb-odm/php-stats#2 shows that ~95% of 2.x users are using at least PHP 7.4 while the minimum required version in 7.2. Given 7.2 -> 7.4 is not bumping for the sake of bumping but would allow us to modernize our code this is a minimum bump we should consider for next minor version.

• Bump minimal PHP version in composer.json
• Remove PHP version checks for PHP 7.x
• Add typed properties and default values where applicable
• Migrate the code to use latest syntax features (arrow functions, etc)

[IonBazan]

By all means! Bumping to 7.4 would give us many syntax improvements but some breaking changes may occur when using covariance/contravariance.

[franmomu]

is anything else missing here? 🤔

It would be nice to release 2.5 to unlock other libraries to add support for new doctrine/* package versions (like DoctrineMongoDBBundle).

We can finish #2489 and leave the removal of annotation dependency for 2.6.

[alcaeus]

No objections from my end. Happy to defer making use of additional 7.4 features (i.e. typed properties) in a future minor version.

[IonBazan]

Closing as implemented in linked PRs.