Move tests to GitHub Actions

[franmomu]

This is based on doctrine/mongodb-odm#2250

I've changed to check just Symfony maintained versions 4.4 and 5.1.

Some doubts:

I don't know if composer global require --no-progress --no-scripts --no-plugins symfony/flex dev-master is necessary.
And also about this:

    # Test dev versions
    - php: 7.3
      if: type = cron
      env: DEV COMPOSER_FLAGS="-n --prefer-dist"

The other cases I think they are covered.

[greg0ire]

Please also update the README badge and .gitattributes

[greg0ire]

The line with symfony/flex was introduced by @alcaeus and @chalasr in 8fcefac as part of #602 . So was the line with cron.

[greg0ire]

@chalasr can you please review this?

[chalasr]

Installing flex globally used to speed up composer package downloads thanks to parallelized prefetching. That's not useful anymore on Composer 2

[greg0ire]

Discussed with @alcaeus about the cron, and we can ignore it for now.

[greg0ire]

Thanks @franmomu !