fix: error when a 'required' value is used on a group or repeat

[cpruijsen]

Title: fix: error when a 'required' value is used on a group or repeat

Closes #856

#### Why is this the best possible solution? Were any other approaches considered?

Conversion now raises an error when a group, repeat or loop has a `required` value. Before, a `required` value on a `begin group` row was written as `<bind nodeset="/data/mygroup" required="true()"/>`, which Collect and Enketo ignore: unlike `relevant`, it does not apply to the questions in the group.

There are two checks:

- `workbook_to_json` in `pyxform/xls2json.py` raises the new `SURVEY_011` error, with the row number, for a `begin group`, `begin repeat` or `begin loop` row that has a `required` value.
- `Section.validate()` in `pyxform/section.py` rejects a `required` bind on a group, repeat or loop. This covers input that does not go through the XLSForm reader (`create_survey_element_from_dict`, `xform2json`).

An explicit false value (`no`, `false`, `false()` and the other false values in `aliases.yes_no`) is still accepted, because it did not produce a misleading bind. Any other value, including an XPath expression, is rejected.

Judgement calls:
- Repeats and loops are rejected too, because they produce the same ignored bind. Limiting this to groups is one condition in each check.
- If you prefer only the XLSForm check, the `section.py` change and `test_group_required_bind__error` can be removed on their own.
- `required_message` on a group is unchanged.

#### What are the regression risks?

Forms that set `required` on a group, repeat or loop to anything other than an explicit false value now fail conversion, as the issue asks. Other forms are unaffected, because the checks only reject input.

#### Does this change require updates to documentation? If so, please file an issue [here](https://github.com/XLSForm/xlsform.github.io) and include the link below.

No. The XLSForm documentation does not list `required` as a group setting.

#### Before submitting this PR, please make sure you have:
- [x] included test cases for core behavior and edge cases in `tests`
- [x] run `python -m unittest` and verified all tests pass
- [x] run `ruff format pyxform tests` and `ruff check pyxform tests` to lint code
- [x] verified that any code or assets from external sources are properly credited in comments