Use strk as a default --fee-token

[cptartur]

Currently, user is required to provide --fee-token or --version everytime they make a transaction.
This is impractical in terms of UX.
Additionally, txs supporting eth fee token are most likely to be removed in the upcoming future https://community.starknet.io/t/snip-16-deprecation-of-transaction-versions-0-1-2/114443

Use strk as a default --fee-token in sncast operations that require providing it.

Additionally, documentation should be updated to stop showing the --fee-token or --version argument except for the "Fees and Versions" page.