d/upstream/signing-key.asc: Add the 5.7 RM's signing key.

I retrieved the key with `gpg --recv-keys 7CA7ECAAF06216B90F894146ACF8146CAE8CBBC4`,
where the hash value was obtained by pulling upstream's zsh-web.git over an SSH
remote and inspecting Arc/source.html in the resulting clone.