Do not allow special characters in new group/namespace path

After !80055 (merged), we do not allow special characters at the start or end of the project path. The project path can neither contain consecutive special characters.

To prevent the issue described in #27483, we need to add the same validation to Group and Namespace models.

Note: adding the validation to the Namespace model will also restrict username.

Availability and Testing

• No new E2E tests or updates needed at this time
• If this feature is released with a feature flag (similar to restrict_special_characters_in_project_path for projects), this should trigger two e2e:package-and-test jobs: ee:instance-parallel, which runs E2E tests with the default feature flag state, and ee:instance-parallel-ff-inverse, which runs E2E tests with the inverse feature flag value.
  • If no feature flag will be used, please trigger the e2e:package-and-test job manually on the MR