[MongoDB] Upgrade driver to 1.21.4 and 2.1.7

[Frzk]

https://github.com/mongodb/mongo-php-driver/releases/tag/2.0.0
https://github.com/mongodb/mongo-php-driver/releases/tag/2.1.0

[EtienneM]

https://github.com/mongodb/mongo-php-driver/releases/tag/2.1.1

[EtienneM]

@Frzk you paused this issue. Could you explain the reason why it is paused?

[Frzk]

We want to support both 1.x and 2.x versions of the extension, mostly because 2.x introduced some breaking changes.
Currently the buildpack doesn't allow us to do that. I'll have to update it to support specifying a precise version of an extension.