[electrum] update servers list

Summary:
Remove a server that has been offline for several months.

Change the URL for fulcrum.pepipierre.cash (same server, new URL, old url is a redirection).

Add two new Chronik Electrum servers.

electrum.bitcoinabc.org remains unchanged, so all previous Electrum ABC releases should keep working without having to manually change the server.

Test Plan:
check in the Network dialog in Electrum ABC that all these servers are listed and on the correct block height

The ones that serve TCP connections can also be queried in a terminal

echo '{"jsonrpc": "2.0", "method": "blockchain.headers.get_tip", "id": "test"}' | nc <URL> 50001

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18388