[apt] provide pgadmin4.sources instead of pgadmin4.list

Please note that security bugs or issues should be reported to security@pgadmin.org.

Describe the bug

in modern ubuntu/debian repository in sources list have sources format

To Reproduce

Steps to reproduce the behavior:

1. open https://www.pgadmin.org/download/pgadmin-4-apt/
2. see

sudo sh -c 'echo "deb [signed-by=/etc/apt/keyrings/packages-pgadmin-org.gpg] https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/$(lsb_release -cs) pgadmin4 main" > /etc/apt/sources.list.d/pgadmin4.list && apt update'

Fix from me:

wget -qO- https://www.pgadmin.org/static/packages_pgadmin_org.pub | sudo gpg --dearmor -o /usr/share/keyrings/packages-pgadmin-org.gpg
echo -e "Types: deb\nURIs: https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/$(lsb_release -cs)\nSuites: pgadmin4\nComponents: main\nArchitectures: amd64\nSigned-By: /usr/share/keyrings/packages-pgadmin-org.gpg" | sudo tee /etc/apt/sources.list.d/pgadmin4.sources

Please fix it

Open the pgAdmin 4 APT download page at https://www.pgadmin.org/download/pgadmin-4-apt/ and find the source-list installation instructions shown in the issue. Update the instructions to use the provided pgadmin4.sources format, then verify that the rendered commands and key path are consistent and complete.