Fixes rjsf-team#2141 by reimplementing rjsf-team#2142

When the value passed to the `CheckboxesWidget` was a single value rather than an array, things would break in the control
- Updated the `CheckboxesWidget` in all themes but `antd` (which uses simpler logic) to fix ensure the value used in the helper functions is an array
- Added a test in `ArrayField_test.ts` that verifies the fix
- Updated the snapshot in `bootstrap-4` which showed an issue when the value string contained an element of the enumeration value rather than the whole value
- Refactored the common `selectValue` and `deselectValue` functions used in `CheckboxesWidget` into `@rjsf/utils`
- Updated the `CHANGELOG.md` accordingly

…onsDeselectValue()` and `enumOptionsSelectValue()` from all the `CheckboxesWidget` implementations into `@rjsf/utils`