Wrong schema class for default_value in invitation_status field

Problem/Motivation

Default value in field invitation_status is string-typed but should be a sequence.

Steps to reproduce

1. Clean install of Drupal, Group & Ginvite
2. Create a group & enable Ginvite
3. Use https://www.drupal.org/project/config_inspector to review field.field.group_content.[your_test_group]-group_invitation.invitation_status:default_value configuration
4. Resulting in : variable type is integer but applied schema class is Drupal\Core\Config\Schema\Sequence

Proposed resolution

Set the schema class to sequence while saving field default value in src/Plugin/GroupContentEnabler/GroupInvitation.php

Remaining tasks

Create patch

User interface changes

N/A

API changes

N/A

Data model changes

N/A