Add expires_at attribute to gitlab_project_share_group resource

cindyrequested to merge
wscix/terraform-provider-gitlab:add-expires-at-to-project-share-group into main

What does this MR do?

Adds support for the `expires_at` attribute to the `gitlab_project_share_group` resource, allowing users to set an expiration date when sharing a project with a group.

What are the relevant issue numbers?

Open issue #6695

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