Prep for 8.0.9

[jamis]

Proposed release announcement:

Mongoid 8.0.9 is now available. This is the most recent patch release in the 8.0 series, and includes the following bug fixes.

• MONGOID-5836 - Callbacks were being duplicated on deeply embedded children. A related issue (MONGOID-5542) was also backported to 8.0, exposing the Mongoid.prevent_multiple_calls_of_embedded_callbacks setting to preserve backwards compatibility in most cases (defaults to false).
• MONGOID-5757 - Child validations were being short-circuited, resulting in subsequent validation callbacks not being called after the first failing validation.
• MONGOID-5797 - Accessing the parent document from an embedded document was failing with an error when the original query applied a projection.
• MONGOID-5810 - When Mongoid.legacy_attributes is true, the #as_document method was returning a hash that leaked internal model state. The hash is now deep-duplicated before being returned (which may have performance implications for large documents or complex hashes).
• MONGOID-5839 - When using single-collection inheritance, eager loading (with #includes) was not producing the correct query when the root of the query was the document subclass.
• MONGOID-5825 - The Mongoid::Timestamps module would (in certain cases) attempt to timestamp deleted documents, which resulted in a FrozenError being raised.