MAINT: Migrate to standard version control procedures

[larsoner]

It's a bit weird to me to have recipes/mne_python_*.*/ rather than a single recipes/mne_python/.

• Most old recipes very well might not work anymore.
• I don't think we'll ever really support multiple versions simultaneously, but even if we do it seems more standard to do this with branches (like maint/1.5 and maint/1.6) than it does with folders.
• Having multiple versions ruins our git blame for lines (for example our latest construct.yaml only has changes going back ~3mo even though some lines are effectively much older)

@hoechenberger if you agree with migrating to a more standard version control scheme under a single recipes/mne_python/, we could try to migrate gracefully by merging backward (rename 0.23 to root mne_python, then merge 0.24 into mne_python, then 0.25 into 0.23, etc. in single PRs). That way we at least get to see what was changed in each version release. (I'll get all git blame in terms of authorship but at least it'll be clearer when stuff was changed.)

[hoechenberger]

@larsoner I'm also happy with just declaring the latest recipe "the" recipe and deleting the old ones, to make things easier… in any case, +1 to having only a single recipe.