Add a horizontal break in logs between flow re-runs

[ecalifornica]

Describe the current behavior

Flow logs do not visually distinguish re-runs.

Describe the proposed behavior

A horizontal rule or some other UI element between each run's logs.

Example Use

No response

Additional context

Less about the use-case, but here are some other logging UI issues I found while searching the backlog:

• https://github.com/PrefectHQ/prefect/issues/8448
• https://github.com/PrefectHQ/prefect/issues/15526