Deprecation plan for STAC versions <1.0

It seems like it would simplify the code base significantly and make long-term maintenance more manageable if we were able to remove support for pre-1.0 STAC spec versions. However, we want to do this in a way that is transparent for users and allows them to plan for updates to libraries that have PySTAC as a dependency. Since removing support for these pre-1.0 versions represents a breaking change, we will also need to consider how it affects versioning of this library. Do we want to remove support for these versions prior to a PySTAC 1.0 release, or should we release 1.0 with deprecation notices for these versions and remove them in a 2.0 release? In either case, we should have a public plan for handling these deprecations in place by the PySTAC 1.0 release.

The purpose of this issue is to discuss a plan for deprecating and subsequently removing support for these pre-1.0 STAC spec versions.

cc: @lossyrob @gadomski @matthewhanson @jbants

Start by reviewing the issue body and its nine-comment discussion about removing pre-1.0 STAC support. Define a public deprecation plan covering whether removal happens before PySTAC 1.0 or in 2.0, and how the breaking change affects versioning. Done means the plan is agreed and documented for users and dependent libraries.