Update vToken minimum minting and redemption values

With the upcoming SLPx protocol vToken, new cross-chain users will be gained. In order to improve users’ small-amount experience, we plan to reduce the minimum value limit for vToken minting and redemption operations. The specific changes are as follows:

vKSM

Mint 0.5 KSM → 0.25 KSM
Redeem 0.4 vKSM → 0.2 vKSM

vMOVR

Mint 0.5 MOVR → 0.25 MOVR
Redeem 0.4 vMOVR → 0.2 vMOVR

vBNC

Mint 1 BNC → 0.5 BNC
Redeem 0.8 vBNC → 0.4 vBNC