Add Dataset#first! to Dataset::ACTION_METHODS

This allows Model.first! to work.
Don't add eager and eager_graph to Model::DATASET_METHODS

These methods should be added by the associations plugin, they
shouldn't be a basic part of Sequel::Model.
Bump version to 3.46.0
Update the 3.46.0 release notes

I noticed that the cache clearing changes can affect backwards
compatibility slightly, so note that and give an example in the
release notes.