Add support for PHP 8

[lepiaf]

depends on

• https://github.com/doesntmattr/mongodb-migrations/pull/51
• :heavy_check_mark: https://github.com/doctrine/DoctrineMongoDBBundle/pull/662
• :heavy_check_mark: https://github.com/doctrine/mongodb-odm/pull/2248

[ivagen]

I am starting a new project in php 8, symfony 5.2 and mongodb. And this pull request would help me a lot. Tell me please. When will php 8 support?

[spantaleev]

Any chance of this getting merged and tagged?