Django-opensearch-dsl

* `Document.update()` method now take an optional `using` argument allowing to specify an alternate
OpenSearch connection defined in `OPENSEARCH_DSL`.
* Fix related document automatic indexation and deletion
(Contributed by [Colin Seifer](https://github.com/Colin-Seifer)).
* Add `pre-delete` back into `BaseSignalProcessor.handle_m2m_changed()` to properly update the
index on M2M interactions (Contributed by [Colin Seifer](https://github.com/Colin-Seifer)).

* Add support for related models. See [Document Classes](document.mddjango-subclass) and
[Document Field Reference](fields.mdusing-prepare_field_with_related) for more information
(Contributed by [Colin Seifer](https://github.com/Colin-Seifer)).
* `django-opensearch-dsl` now only tests supported version of Python and Django (mainstream and LTS).
This choice is made to:
* Speed up development.
* Speed up tests.
* Reduce actions on github.
* Encourage people to update their stack to supported (thus safer) versions.
* Drop support for Python 3.6.
* Drop support for Django 2.1, 2.2, 3.1.
* Now supports Django 4.1.
* Now supports `opensearch-dsl>=2.0 <3.00`.

* Fixes internal links in documentation.
* Removes the need to declare a `TESTING` boolean in `settings.py`.