Bump PoV request timeout (#5924)

# Description

We previously set the PoV request timeout to 1.2s based on synchronous
backing, which allowed for 5 PoVs per relay block. With asynchronous
backing, we no longer have a time budget and can increase the value to
2s.

Fixes https://github.com/paritytech/polkadot-sdk/issues/5885

## Integration

This PR shouldn't affect downstream projects.

## Review Notes

This PR can be followed by experiments with Gluttons on Kusama to
confirm that the timeout is sufficient.