C#: Fix crash with DisposablesTracker_OnGodotShuttingDown

[RedworkDE]

Fixes #77486
Also Fixes #77305

As described in the issue, because DisposablesTracker_OnGodotShuttingDown is called after getting rid of all script bindings, if deleting an object also deletes other objects that are later in the queue of managed objects to delete, the later managed object isn't marked as deleted and later a use-after-free occurs.
Thus move the DisposablesTracker_OnGodotShuttingDown call to before deleting the script bindings.
(Also the finalizing member is probably no longer needed, but I didn't test that)
MRP for the original issue: Test_77486_min.zip (ASAN only, simply start the project and close it again)

While I don't think this ha any side-effects beyond a slightly degraded shutdown performance, @neikeq should probably review this as there may be reasons to do things in the previous order that I missed.

Alternatively the root cause is that the managed shutdown deletes objects that are not owned by the managed side, but there is no way to check this.

[raulsntos]

The change makes sense to me, but I couldn't reproduce the issue.

As for why the call to OnGodotShuttingDown was in the GDMono destructor. The DisposablesTracker was added in f88d890 to replace the old Mono cleanup.

I don't think the previous order was important, I think OnGodotShutdown just happend to be added there because we were replacing the Mono cleanup which naturally happened in the GDMono destructor. So it's probably fine to change the order.

Since I couldn't reproduce the issue it makes me feel uneasy about approving, and normally I'd prefer to move it to 4.3 so we can get more testing, but it seems we may need this PR and #84013 to fix an important regression in 4.2 (#82279). Also, I trust that @RedworkDE has tested it. So I'll approve the PR.

[akien-mga]

Thanks!