[MAINT] Run CIs on MNE-stable and update to latest Python version

[tsbinns]

Suggested by @larsoner. Currently only run unit tests on mne-main which can lead to bugs slipping through (e.g. #286).

This adds a test for the stable MNE version on Ubuntu. Should it be tested on more platforms like in MNE-BIDS?

Also updates the latest Python version tested from 3.12 to 3.13.

[tsbinns]

Also, should this be updated to a Python version we're actually testing or just remove it?

mne-connectivity/.github/workflows/unit_tests.yml

Lines 68 to 69 in bccdceb

	- run: python -c "import mne; print(mne.datasets.testing.data_path(verbose=True))"
	if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.11'

[larsoner]

Also, should this be updated to a Python version we're actually testing or just remove it?

Ouch yes it means we've accidentally been skipping some tests I think. I would actually just get rid of the conditional, I think we always want the testing dataset