Change default CatalogType to RELATIVE_PUBLISHED

[gadomski]

We end up with a lot of folks expecting relative hrefs instead of absolute:

• Collection link points to local path #838
• Adding custom links between sibling catalogs creates absolute HREFs #1112

I'm proposing to change our default catalog type

pystac/pystac/catalog.py

Line 184 in 0ea4786

catalog_type: CatalogType = CatalogType.ABSOLUTE_PUBLISHED,

to RELATIVE_PUBLISHED, which will likely fix both of these issues

I think this would be considered a breaking change, as folks might be expecting absolute behavior by default? Because of that, I'm not opening a PR to do it, but rather opening this issue to build consensus on whether (a) we should do this and (b) if this should be part of a MAJOR release (instead of a MINOR).