Enable delayed deletion for personal namespace projects

Problem To Solve

We want to make delayed project deletion available for projects in user namespaces (personal projects). Previously, this safeguard against accidental data loss was only available for group namespaces.

When you delete a project in your user namespace, it should enter a "pending deletion" state for the duration configured in your instance settings (7 days on GitLab.com), rather than being immediately deleted. This creates a recovery window during which you can restore the project if needed.

Context

This was enabled briefly in the past with Add support for delayed project deletion for pe... (!89466 - merged) but was reverted with Disable delayed project deletion feature for pe... (!95495 - merged), due to:

• https://gitlab.com/gitlab-org/gitlab/-/issues/369077+
• Can't fork project after deleting fork (#369498 - closed)

I investigated these issues and came to the following conclusions:

1. The main problem from the first issue was that pending deletion projects were still counting towards quotas. I found out this has been solved since then, see #534486 (comment 2451241789).
2. The other issue about forking doesn't seem to exist anymore, see #369498 (comment 2450219868).
3. The other problems reported at https://gitlab.com/gitlab-org/gitlab/-/issues/369077+ were mostly UI/consistency minor issues that should be easy to solve and shouldn't be blockers to enable this.