Do not enforce PoV size hard limit in config (#5887)

Quoting @bkchr (from
[here](https://github.com/paritytech/polkadot-sdk/issues/5334#issuecomment-2383815078)):

> That the hardcoded limit is used there again, is IMO not correct. The
`max_pov_size` should be controlled by the `HostConfiguration` and not
limited by some "random" constant.

This PR aims to change the hard limit to a not-so-random constant,
allowing more room for maneuvering in the future.