.NET Debian packages priority update

PMC team received an [issue](https://github.com/microsoft/linux-package-repositories/issues/23) of .NET debian packages having priority set as "standard" instead of "optional".  Based on the customer issue, PMC team, and Debian [manual](https://www.debian.org/doc/debian-policy/ch-archive.html#s-priorities), all the packages but the ones installed default with the Debian installation must set priority of packages as "optional". This priority is set in packages Metadata.

Packages reported to check from PMC team -
"aspnetcore-runtime-3.1"
"aspnetcore-runtime-6.0"
"aspnetcore-runtime-7.0"
"aspnetcore-targeting-pack-6.0"
"aspnetcore-targeting-pack-7.0"
"dotnet-apphost-pack-3.1"
"dotnet-apphost-pack-6.0"
"dotnet-apphost-pack-7.0"
"dotnet-host"
"dotnet-hostfxr-3.1"
"dotnet-hostfxr-6.0"
"dotnet-hostfxr-7.0"
"dotnet-runtime-3.1"
"dotnet-runtime-6.0"
"dotnet-runtime-7.0"
"dotnet-runtime-deps-3.1"
"dotnet-runtime-deps-6.0"
"dotnet-runtime-deps-7.0"
"dotnet-sdk-3.1"
"dotnet-sdk-6.0"
"dotnet-sdk-7.0"
"dotnet-targeting-pack-6.0"
"dotnet-targeting-pack-7.0"