Increase maximum number of characters in asset symbol to 5

[joemarct]

It's not uncommon for tokens/cryptos in other blockchains to have 5-character symbols. Examples are WAVES, OCEAN, SUSHI, CREAM, STORJ. You'll find a handful more if you scan CoinMarketCap. There are token symbols that have more than 5 characters but uncommon. I think increasing the limit to 5 for X-Chain assets is still reasonable.

My motivation for requesting this change is that we have a token project in another blockchain with 5-character symbol we'd like to move it to Avalanche. I can imagine that other projects are (or will be) in the same situation.