Support group_content_string_id module on group_webform

Problem/Motivation

When the module is used with the patch on ✨ Entities identified by strings as group content Closed: won't fix it works as expected, but when used with the group_content_string_id → (the recommended approach by the maintainer of the group module*), there is a fatal error when a non-administrator user tries to access to an individual group webform:
Error: Call to undefined method Drupal\group\Entity\GroupContent::getEntityFieldNameForEntityType() in Drupal\group_webform\GroupWebformService->webformAccess() (line 110 of modules/contrib/group_webform/src/GroupWebformService.php).

Steps to reproduce

• Install group_taxonomy and group_content_string_id modules
• Create a group, and add a user as member of that group
• Add a webform to a group
• Access to that webform

Proposed resolution

Avoid using the GroupContent::getEntityFieldNameForEntityType() function

Remaining tasks

• Patch
• Review
• Test

User interface changes

None

API changes

- Patch on ✨ Entities identified by strings as group content Closed: won't fix is replaced by the group_content_string_id module

Data model changes

- entity_id_str column in the group_conent_field_data table is removed
- entity_id column in the group_conent_field_data table now a varchar field
- Both changes are managed by the group_content_string_id module

Notes

* See https://www.drupal.org/project/group/issues/2797793#comment-14149767 ✨ Entities identified by strings as group content Closed: won't fix