Simplify public key download for debian repository install

In https://adoptium.net/installation/linux/#_deb_installation_on_debian_or_ubuntu , point 2, following command is used :
```
wget -qO - https://packages.adoptium.net/artifactory/api/gpg/key/public | gpg --dearmor | tee /etc/apt/trusted.gpg.d/adoptium.gpg > /dev/null
```

This could be simplified by using a *.asc file :
```
wget -qO - https://packages.adoptium.net/artifactory/api/gpg/key/public > /etc/apt/trusted.gpg.d/adoptium.asc
```

Context : when && chaining commands in a dockerfile RUN statement, pipes are difficult to work with.

Start at the Debian or Ubuntu installation page linked in the issue and locate point 2 of the deb installation instructions. Update the public-key command as described, then verify that the rendered instructions show the simplified command and still cover the repository installation steps.

Written by the indexing model from the issue text.