Stricter defaults for concat, combine, open_mfdataset, merge

Is your feature request related to a problem?

The defaults for concat are excessively permissive: data_vars="all", coords="different", compat="no_conflicts", join="outer". This comment illustrates why this can be hard to predict or understand: a seemingly unrelated option decode_cf controls whether a variable is in data_vars or coords, and can result in wildly different concatenation behaviour.

1. This always concatenates data_vars along concat_dim even if they did not have that dimension to begin with.
2. If the same coordinate var exists in different datasets/files, they will be sequentially compared for equality to decide whether they get concatenated.
3. The outer join (applied along all dimensions that are not concat_dim) can result in very large datasets due to small floating points differences in the indexes, and also questionable behaviour with staggered grid datasets.
4. "no_conflicts" basically picks the first not-NaN value after aligning all datasets, but is quite slow (we should be using duck_array_ops.nanfirst here I think).

While "convenient" this really just makes the default experience quite bad with hard-to-understand slowdowns.

Describe the solution you'd like

I propose we migrate to data_vars="minimal", coords="minimal", join="exact", compat="override". This should

1. only concatenate data_vars and coords variables when they already have concat_dim.
2. For any variables that do not have concat_dim, it will blindly pick them from the first file.
3. join="exact" will prevent ballooning of dimension sizes due to floating point inequalities.
4. These options will totally avoid any data reads unless explicitly requested by the user.

Unfortunately, this has a pretty big blast radius so we'd need a long deprecation cycle.

Describe alternatives you've considered

No response

Additional context

xref https://github.com/pydata/xarray/issues/4824
xref https://github.com/pydata/xarray/issues/1385
xref https://github.com/pydata/xarray/issues/8231
xref https://github.com/pydata/xarray/issues/5381
xref https://github.com/pydata/xarray/issues/2064
xref https://github.com/pydata/xarray/issues/2217

Start by comparing the current defaults and behavior of concat, combine, open_mfdataset, and merge, then read the linked issues #4824, #1385, #8231, #5381, #2064, and #2217 for prior context. The proposed work requires a deprecation cycle; it is done when the stricter defaults and their migration path are agreed and applied consistently.