Add skip_wait_for_deletion attribute to gitlab_group_service_account

What does this MR do?

Adds an optional skip_wait_for_deletion boolean attribute to the gitlab_group_service_account resource. When set to true, the provider skips waiting for the service account to be fully deleted after calling the delete API.

What are the relevant issue numbers?

Closes #6809 (closed)

Implementation Details

• Added skip_wait_for_deletion boolean attribute to the resource schema (optional, defaults to false)
• Updated the Delete function to check this attribute and skip the polling logic when true
• Added test case TestAcc_GitlabGroupServiceAccount_skipWaitForDeletion to verify the functionality
• Documentation auto-generated with recommendation to use this for gitlab.com

Usage Example

resource "gitlab_group_service_account" "example" {
  group                  = "my-group"
  name                   = "service-account"
  skip_wait_for_deletion = true  # Recommended for gitlab.com
}