Update pallet_uniques string limits.

[peterwht]

Motivation
The current pallet-uniques configuration for StringLimit is too low, at 50 bytes. Users of uniques often store the IPFS CID in the metadata. These CIDs are larger than 50 bytes.

Furthemore, the ValueLimit seems too high at 256 bytes. The value applies to attributes, which would rarely reach 256 bytes.

Suggested Solution

I recommend updating the configuration to use the same as Polkadot's asset hub.
https://github.com/paritytech/polkadot-sdk/blob/master/cumulus/parachains/runtimes/assets/asset-hub-polkadot/src/lib.rs#L683

• StringLimit = 128
• KeyLimit = 32
• ValueLimit = 64

[stiiifff]

Closed by #298