MONGOID-5228 disallow _id to be updated on persisted documents (#5542)

* MONGOID-5228 - disallow _id to be updated on persisted documents

Previously, updating _id on any document (top-level or embedded) would silently ignore the _id attribute, making it appear as though the operation succeeded. This was misleading, since the _id didn't actually change. This change makes it so that _id is explicitly made immutable once a document has been persisted. The same behavior is intentionally enforced regardless of whether the document is embedded or not. If you find yourself needing to update the _id field in an embedded document, you'll need to use the lower-level driver methods to accomplish this. We believe modifying the _id field (even in embedded documents) is an anti-pattern.

* use a specialized error for immutability violations

* add a feature flag

* change the flag name, and account for existing nested attribute behavior

* wrong default for <9.0

* clean up a test that was unnecessarily sensitive to the _id changing

* another test that was unnecessarily sensitive to _id changes

* more tests that are too sensitive to _id changes

* we need to allow _id mutations if the _id was previously nil

* mention enforcement of _id immutability in the release notes

Author: jamis

source/release-notes/mongoid-9.0.txt

@@ -204,6 +204,25 @@ This means that, by default, Mongoid 9 will update the existing document and
 will not replace it.
 
 
+The immutability of the ``_id`` field is now enforced
+-----------------------------------------------------
+
+Prior to Mongoid 9.0, mutating the ``_id`` field behaved inconsistently
+depending on whether the document was top-level or embedded, and depending on
+how the update was performed. As of 9.0, changing the ``_id`` field will now
+raise an exception when the document is saved, if the document had been
+previously persisted.
+
+Mongoid 9.0 also introduces a new feature flag, ``immutable_ids``, which
+defaults to ``true``.
+
+.. code-block:: ruby
+
+  Mongoid::Config.immutable_ids = true
+
+When set to false, the older, inconsistent behavior is restored.
+
+
 Bug Fixes and Improvements
 --------------------------