remove old single gpg signature binary verification process and update with multi signature approach

[AndySchroder]

In the full node setup,

https://github.com/bitcoin-dot-org/Bitcoin.org/blob/e324324705b48be459f936e210222ba8441a339c/en/full-node.md?plain=1#L36

it says that binary files are signed with a single release signature, but now they are signed with multiple signatures as is described on https://bitcoincore.org/en/download/ .

[AndySchroder]

Actually, looks like this is a duplicate of https://github.com/bitcoin-dot-org/Bitcoin.org/issues/3781 .