Note deprecation of the project level locales in the docs

[Jannis Leidel]

We've started to deprecate the project level locales in 1.3 but forgot to add it to the deprecation docs; the warning is already a DeprecationWarning so we better add it soon: https://code.djangoproject.com/browser/django/trunk/django/utils/translation/__init__.py#L52

[Ramiro Morales]

In [17394]:

Added note about deprecation of project-level translations to the deprecation timeline document.

Thanks Jannis for the report. Fixes #17588.

[Ramiro Morales]

In [17395]:

[1.3.X] Added note about deprecation of project-level translations to the deprecation timeline document.

Thanks Jannis for the report. Fixes #17588.

Backport of [17394] from trunk.