* Remove unused code path in MongoDBFormatter
Monolog already required mongodb/mongodb 1.8+ (and a related ext-mongodb version) so this code path was never used.
* Use Collection::getCollection() from mongodb/mongodb 1.21
The original selectCollection method is deprecated. Since Monolog itself requires PHP 8.1, it should be sane to rely on 1.21+, which shares the same requirement.
* Update class refs for MongoDB extension and library
* Conditionally use Collection getter by version
* Relax PHPLIB version requirement
* Use RequiresPhpExtension attribute to require ext-mongodb
* Import UTCDateTime class and revise var names
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