Migrate by default

[gadomski]

As described in #1503 (comment), our from_file method migrates to the latest version by default, but from_dict does not. This can lead to confusing discrepancies in behavior depending on how STAC data gets into pystac. I think we should align on migrate-by-default, as we will continue to see more and more STAC (and extension) versions as time goes on.

This could be as simple as change the from_dict default. This could lead to unexpected behavior for some users but IMO that'd be worth it, and not a breaking change. Thoughts?

[jsignell]

Yeah I like migrate by default. People can stay on an old version of pystac if they want to be using an old API for interacting with their STAC