Revert "Fixed #23384 -- Allowed overriding part of a dictionary-type setting"

This reverts commit 66757fee7e921ad4c35e0b3f80c25e026100b31c.
Discussions have led to think that this functionality does not
bring significant benefits to justify the added complexity.
Read also discussions on ticket #22734.
Fixed typo in docs/topics/checks.txt.
Fixed #22534: Reinforce swappable documentation
Removed an admonition in the tutorial about the old project layout.

There are no supported versions that use the old layout (<=1.3).
Fixed #22411: Throw a more helpful error if contenttypes doesn't exist.
Fixed #23414 -- Updated version numbers in release process doc.

Thanks jaap3 for the report.
Update docs to be more clear about nested classes (refs #23418)
 switch out recursive dfs for stack based approach, to avoid possibly hitting the recursion limit
Merge pull request #3171 from ClipCard/master

Prevent possible recursion limit issue in migration planning
Fix Python 3 incompatability
Fixed #23427 -- Documented migrate is required before runserver for default projects.
Fixed #23380 -- Fixed flaky test: admin_filters.tests.ListFiltersTests.test_relatedfield
Fixed #23430 -- simplified gunicorn deployment docs.
Fixed #22161 -- Documented send() with no recipients won't raise an exception.
Fixed #23438 -- Added snippet & imports to docs/intro/overview.txt.
Fixed #23340 -- Corrected naturaltime docs to match behavior.

Thanks zachborboa for the report and blackguest for the patch.
Additional edits for refs #23340; thanks Loic.
Fixed #22882 -- Fixed a UnicodeDecodeError in flush.

Thanks djbaldey for the report.
Refs #23430 -- gunicorn wants a module, not file
Fixed #23437 -- Documented required update of WSGI scripts for 1.7.

Thanks Tim for the review.