MONGOID-5744 prep for release of 8.0.8

[jamis]

Proposed release announcement:

Mongoid 8.0.8

This patch release fixes two bugs:

MONGOID-5704: By default, associations (like belongs_to) are validated when a document is saved. However, Mongoid was aggressively loading persisted associations and validating them, which led to a significant performance regression in Mongoid 8+ (versus Mongoid 7.x). This patch fixes this regression by only validating associations that are (1) currently in-memory, and (2) either unpersisted or modified.

MONGOID-5709: has_and_belongs_to_many associations on embedded docments were broken, and attempting to use them would result in an "InvalidPath" exception. This patch release fixes that bug.