Fix benchmarks failing on scheduler overflow

[muharem]

Scheduler has the upper limit for the number of the calls scheduled per block.
The benchmarks of the pallet_referenda and pallet_conviction_voting do not respect the limit and overflows it if the scheduler MaxScheduledPerBlock not high enough.

For pallet_referenda benchmarks the limit reached if
track.max_deciding > scheduler.max_scheduled_per_block OR
referenda.max_queued > scheduler.max_scheduled_per_block OR
track.max_deciding + referenda.max_queued > scheduler.max_scheduled_per_block

For pallet_conviction_voting benchmarks the limit reached if
number_of_registered_votes > scheduler.max_scheduled_per_block

This PR fixes it with aim to make it seamless for users.

[muharem]

I did not expect the solution to be so complex, here the alternative way that only helps a user to find a workaround #13231
cc: @bkchr @ggwpez

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.