[electrum] bump checkpoint to 822000

Summary:
Bumping the checkpoint saves bandwidths for new users (andi thus also for servers), as they need to download the headers from the checkpoint to the tip when they first start the software.

See the doc for the ElectrumX documentation regarding checkpoints:

• https://electrumx.readthedocs.io/en/latest/protocol-basics.html?highlight=merkle#block-headers
• https://electrumx.readthedocs.io/en/latest/protocol-methods.html#blockchain-block-header

Test Plan:
Run Electrum ABC, open the console and type:

network.synchronous_get(("blockchain.block.header", [822000, 822000]))

Check that the new VERIFICATION_BLOCK_MERKLE_ROOT matches the "root" field in the returned dict.
Also check that the "header" field matches the raw header on https://explorer.e.cash/block/00000000000000001f972e21924e8f8ab502dcdc3adc76409dc62b9b393d810d (Advanced block data)

Run the software for a while and check that the local height is updated as new blocks are announced by the servers.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14972