[2.0] Document BC breaks and add deprecations in 1.3

[alcaeus]

For every BC break we made in 2.0, there needs to be a deprecation warning and (if possible/sensible) a forward layer for people to prepare switching to ODM 2.0. This affects, but is not limited to BC breaks in #1553.

[malarzm]

FWIW I was using labels for PRs breaking BC: https://github.com/doctrine/mongodb-odm/issues?utf8=%E2%9C%93&q=label%3A%22BC+break%22+milestone%3A2.0.0

[alcaeus]

BC breaks have been documented in the UPGRADE document. Now we need to add BC layers (deprecations, possibly backports of new classes, etc.) in 1.3. All PRs that need this have been tagged with the needs-1.3-compat-layer tag.

[alcaeus]

Closing as current BC breaks have been documented in 1.3.