Confusing behaviour with autosummary and multiple source_suffix entries.

Describe the bug

It appears that the autosummary generates files with the first extension in source_suffix even though autosummary only generate reStructuredText. This is a problem when using multiple entries in source_suffix for several reasons.

1. If something other than '.rst' is the first entry (say '.myst'), then autosummary will generate a reStructuredText file with a .myst extension, which will then be interpreted differently and not parsed properly.
2. The docs suggest that source_suffix should be a regular dictionary, implying that order is not important, yet autosummary depends on the order. (I realize that for python > 3.7 dictionaries are ordered... but Sphinx currently supports 3.6+, so this is an implementation detail, not a language guarantee).

This is likely related to issue #2485, but the fix there does not seem to work in this case.

How to Reproduce

$ git clone https://github.com/mforbes/sphinx_issue_9891/
$ cd sphinx_issue_9891
$ pip install -r requirements.txt
$ make html
$ open _build/html/_generated/mymod.html

Expected behavior

Since autosummary currently only produces reStructuredText, it should always produce a ".rst" file or similar if that is included in source_suffix, otherwise a warning or error should be raised.

Your project

https://github.com/mforbes/sphinx_issue_9891/

Screenshots

This order works: mymod.html

source_suffix = {".rst": "restructuredtext", ".myst": "myst"}

This order fails: mymod.html because autosummary generates .myst files containing ReStructuredText, and then myst_parser picks these up and tries to interpret them as markdown.

source_suffix = {".myst": "myst", ".rst": "restructuredtext"}

OS

Mac OS X 10.14.6, but most likely cross platform

Python version

3.9

Sphinx version

4.2.0

Sphinx extensions

sphinx.ext.autosummary, myst_parser

Extra tools

make

Run the linked reproduction with the two source_suffix orderings and compare the generated autosummary output. Start by tracing autosummary's handling of source_suffix and the myst_parser/reStructuredText extensions. Done means generation is independent of dictionary order, or an appropriate warning or error is produced when .rst is unavailable.