Drupal 9 compatibility

[stevenlafl]

Drupal 9 requires Symfony 4. We are currently locked at Symfony 3.

 - Conclusion: don't install acquia/acsf-tools 9.0.0
    - drupal/core-recommended 9.0.0 requires symfony/yaml v4.4.9 -> satisfiable by symfony/yaml[v4.4.9].

This updates the requirement to use Symfony 4.

[sdelbosc]

Thanks @stevenlafl.
Would you mind confirming

• that you got a chance to successfully test the change you raised on that PR
• whether or not your project uses Acquia BLT and if which version
• whether or not your project uses config from acsf_tools_config.yml

[nikunjkotecha]

IMO we should close this in favor of #119

[sdelbosc]

Would be better to tackle this in a similar manner as what had been done with #118