Use `deleted_at` column to mark groups in deletion process

What does this MR do and why?

The current 'pending_delete' column name is misleading. It suggests future deletion but actually indicates an ongoing deletion process. The new column name better reflects the actual state of the group and avoids confusion about the deletion status.

Here we don't need to backfill this column because the only finder using the current column i.e. Organizations::GroupsFinder is experimental.

Query plans

Before: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29629/commands/91974

After: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/33105/commands/102254

References

#501158

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Related to #501158