Bump version to 9.0.1

[adviti-mishra]

Bumped the version of Mongoid from 9.0.0 to 9.0.1 in accordance with Semantic versioning.

Here is a draft of the release notes:

9.0.1 release notes

Version 9.0.1 of Mongoid is now available.

Release Highlights

• MONGOID-5786: In accordance with Ruby's enumerable API, the #sum method now accepts an optional block. Thank you to Cristián Pérez for your contributions!
• MONGOID-5688: Problematic recursive callstacks in cascading callbacks have been linearized using Ruby Fibers to prevent SystemStackErrors with greater numbers of embedded documents. Thank you to Adviti Mishra for your contributions!
• MONGOID-5769: $pop and $pull are now mongoized the same way as #addToSet or $push to ensure update_all functions as intended. Thank you to John Maguir and Michael Deryugin for your contributions!
• MONGOID-5789: querying an attribute with a nil name now returns an empty string, rather than raising an exception. Thank you to Dan Healy for your contribution!
• MONGOID-5785: if you set Mongoid.allow_scopes_to_unset_default_scope = true, you can invoke (e.g.) unscoped in a named scope to reset the current scope. This is useful for overriding default scopes inside of a named scope. This option will default to true in Mongoid 10.
• MONGOID-5791: If the parent document class is not loaded at the time of loading the embedded document class, a NameError used to be raised. This has been fixed.
• MONGOID-5796: The docs previously present in the Mongoid repository have been moved to the docs-mongoid repository.

[jamis]

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