fix(cli): use [signed-by] instead of the deprecated apt-key command for apt-based install

[sbellone]

Description

apt-key is deprecated and will disappear in the next releases of Debian and Ubuntu as mentioned in the man page.

The recommended way is now to use signed-by in the sources.list entry: https://wiki.debian.org/DebianRepository/UseThirdParty

This PR updates the apt-based install script to use this new method.

Test

I've run this updated script on my machine, I now have the heroku key in /usr/share/keyrings/heroku-archive-keyring.gpg and the install was performed as expected:

$ sh install-ubuntu.sh 
This script requires superuser access to install apt packages.
You will be prompted for your password by sudo.
[sudo] password for <user>: 
+ dpkg -s apt-transport-https
+ echo deb [signed-by=/usr/share/keyrings/heroku-archive-keyring.gpg] https://cli-assets.heroku.com/apt ./
+ dpkg -s heroku-toolbelt
+ true
+ curl https://cli-assets.heroku.com/apt/release.key
+ sudo gpg --dearmor -o /usr/share/keyrings/heroku-archive-keyring.gpg
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  1753  100  1753    0     0  18849      0 --:--:-- --:--:-- --:--:-- 18849
+ apt-get update
Hit:1 http://deb.debian.org/debian bullseye InRelease
Hit:2 http://deb.debian.org/debian bullseye-updates InRelease                                                                                                   
Hit:3 http://security.debian.org/debian-security bullseye-security InRelease                                                                                    
[...]                                                                             
Get:7 https://cli-assets.heroku.com/apt ./ InRelease [2,550 B]                   
[...]
Get:11 https://cli-assets.heroku.com/apt ./ Packages [620 B]
Fetched 9,921 B in 1s (10.7 kB/s)
Reading package lists... Done
+ apt-get install -y heroku
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following NEW packages will be installed:
  heroku
0 upgraded, 1 newly installed, 0 to remove and 23 not upgraded.
Need to get 28.0 MB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 https://cli-assets.heroku.com/apt ./ heroku 7.60.2-1 [28.0 MB]
Fetched 28.0 MB in 1s (19.7 MB/s)  
Selecting previously unselected package heroku.
(Reading database ... 233783 files and directories currently installed.)
Preparing to unpack .../heroku_7.60.2-1_amd64.deb ...
Unpacking heroku (7.60.2-1) ...
Setting up heroku (7.60.2-1) ...
heroku installed to /usr/bin/heroku
 ›   Warning: Our terms of service have changed: https://dashboard.heroku.com/terms-of-service
heroku/7.60.2 linux-x64 node-v14.19.0

[vpavic]

Could someone from the Heroku team take a look at this and hopefully merge it?

Using the Ubuntu install script to install the CLI in the last couple of Ubuntu releases results in the following warning when running apt update:

W: https://cli-assets.heroku.com/apt/./InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.