miniflare: old Pool not closed when runtime entry URL changes

When `previousEntryURL` differs from the new `#runtimeEntryURL` at packages/miniflare/src/index.ts:2143-2150, a new Pool is assigned to `this.#runtimeDispatcher` without first closing/destroying the previous Pool instance. This is a minor resource leak — the old Pool's connections will eventually time out, but it would be cleaner to explicitly close it. The `dispose()` method at line 2791 also doesn't explicitly close the dispatcher, though workerd shutting down implicitly terminates the connections.