Add deprecation warnings for showCancelButton

Editor assessment

The subject states the deprecation work exactly: warnings were added for the showCancelButton option in the searchbar, which tells maintainers the option is on its way out without any behavior yet removed. refactor(searchbar) is a defensible type for warning instrumentation, and the scope is exact. No body or footer, and while pointing at the replacement option would help users, the pull number gives provenance. Wording is compact and clean. Proportionate for a deprecation step.