migrated to netstandard2.0, added support for net7.0, postgres 11+

[f1nzer]

There are several changes:

• library migrated to netstandard2.0, because there is no reason to use any different target
• dropped support for net5.0 - it is not supported anymore
• some functions are now stored procedures, so postgres 11+ is supported
• added support for net7.0 - fixes #31
• removed error log in case of duplicates - this situation shouldn't raise errors, there is nothing special about that

I've run your samples and everything works, but also please test the lib by yourself.

[sergey-tihon]

@f1nzer probably a few functions should be replaced by procedures in README.md as well

[sergey-tihon]

@leonibr can you please release this (at least as a prerelease package)? This is the last dependency that stops us from using .NET 7.

[wub]

Hey @f1nzer and @leonibr, we can no longer use this package with Npgsql under v7 after this change. Am I doing something wrong? Thank you.

error NU1605: Detected package downgrade: Npgsql from 7.0.0 to 6.0.7. Reference the package directly from the project to select a different version. 
/x.csproj : error NU1605:  X.Host -> Community.Microsoft.Extensions.Caching.PostgreSql 4.0.1 -> Npgsql (>= 7.0.0) 
/x.csproj : error NU1605:  X.Host -> Npgsql (>= 6.0.7)

[leonibr]

Your X.Host is using Npgsql 6.0.7, update that to(>= 7.0.0) as well

[wub]

@leonibr Yes you are correct, however we were hoping to avoid bumping the major version of Npgsql just for this one package, as we have a range of other packages still using 6.

I guess we'll take this opportunity to move everything else to v7. Thank you.