Don't overwrite indexes for region writes, always

[dcherian]

What happened?

Currently we don't overwrite indexes when region="auto"

xarray/xarray/backends/api.py

Lines 1769 to 1770 in e6ccedb

	if region_was_autodetected:
	dataset = dataset.drop_vars(dataset.indexes)

I propose we do this for all region writes and completely disallow modifying indexes with a region write.

This would match the map_blocks model, where all indexes are specified in the template and no changes by the mapped function are allowed.

[max-sixty]

Very much agree!

IIRC, currently it's disallowed to write indexes that don't contain the region (writing any var that doesn't contain the region is disallowed)

So this is only an issue for indexes which do contain the region. Historically the index would generally have been dropped manually; makes sense to exclude it.

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(I also thought about whether this was an issue for other variables which had fewer bigger chunks. But actually this is disallowed in zarr, it's only the index that can have different chunks)

[slevang]

There was a bunch of discussion about this on #8434 - I'm in favor. I decided to take the smaller step on that PR for only region="auto" writes (where this is definitely not safe for distributed use) because I hadn't thought through all the edge cases.