Deprecation of `astar-sepolia-testnet`

[hiletmis]

Astar testnet has been deprecated. It is already retired from market but it is still in @api3/chains package. As there is no such chain ATM there are two possible action that can be taken:

• Chain configuration removal
• Checking skipProviderCheck flag

[hiletmis]

@bbenligiray for visibility.

[bbenligiray]

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