Rename public key published in releases

[abailly-iohk]

Why

Most OSes come with tools that understand GPG keys and can automatically add them to a keyring, but this is often based on filenames' extension. The .key extension is too generic and (at least on Mac OS X) not recognised as a public key.

Changing the extension makes it easier for end-users to add the key to their rings in one-click.

What

Change the name of the public key in the releases page from gpg-public.key to public-key.gpg.