Promote delta time debug log to warning

## What problem does this solve or what need does it fill?

In https://github.com/bevyengine/bevy/pull/10145 I demoted this to a `debug!` log because it is regularly triggered when doing things like loading scenes.

This _is_ a useful warning (as it is an indicator of doing too much work in Update / it means a hitch occurred), but it is currently unactionable for this common case, which is why we opted to disable it.

## What solution would you like?

Ideally we can move the work from our scene / rendering code that causes this hitch out of Update and onto a separate thread. This would both solve a hitching problem and allow us to promote this log back to a warning without spamming peoples' consoles.

Start by tracing the delta-time debug log and the scene and rendering work triggered from Update. Reproduce the hitch while loading scenes, then identify which work can be moved to a separate thread without breaking rendering or scene loading. Done means the common case no longer hitches in Update and the diagnostic can be promoted to a warning without console spam.

Written by the indexing model from the issue text.