exp show: make --only-changed default

[dberenbaum]

exp show --only-changed seems like the most useful way to show experiments in most scenarios, and some questions we get about filtering the columns seem like they would be resolved by using --only-changed. Some background:

• https://github.com/iterative/vscode-dvc/issues/1994#issuecomment-1177717853
• https://github.com/iterative/dvc/issues/6271

This would mean needing swapping flags and introducing something like --unchanged. Need to consider whether this should wait for a major release (exp functionality is experimental in 2.x but want to avoid breaking workflows and other products).