Rename GPG public key file to use .gpg extension

[phanect]

#834

(I avoid Closes keyword this time)

Description

Some versions of apt may assume that any .asc key is ASCII-armored. Therefore, if GPG public key has .asc extension, some errors raised on apt update and you cannot install github-desktop.

W: GPG error: https://apt.packages.shiftkey.dev/ubuntu any InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7FC979028B1997C1
E: The repository 'https://apt.packages.shiftkey.dev/ubuntu any InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

By changing the extension, this error is not raised.

See: #834 (comment)

Screenshots

N/A

Release notes

Notes:

[shiftkey]

Thanks for the assistance with digging into this issue!