Remove support for doctrine/mongodb-odm 1.x

[franmomu]

Ref: https://github.com/sonata-project/exporter/pull/375#discussion_r492254857

Right now, we are supporting in some repositories doctrine/mongodb-odm 1.x and 2.x. We could drop support for 1.x since:

• 1.x uses the deprecated mongodb PHP extension, if we drop 1.x there is no need of alcaeus/mongo-php-adapter.
• 2.0 was release 1 year ago
• 2.0 requires minimum PHP 7.2 while 1.x requires minimum of 5.6 (2.x can use the features of php +7.2).
• We can get rid of some variants in dev-kit

These are the repositories that have mongodb service in dev-kit:

• [x] classification-bundle
• [x] doctrine-extensions
• [x] doctrine-mongodb-admin-bundle
• [x] exporter
• [x] media-bundle
• [x] dev-kit variants

[franmomu]

Done!