Fix many many warnings... and bump up .NET version to .NET 6

What's Changed

• SYSLIB の警告を除去 by @devlights in #28
• Add pragma disable by @devlights in #30
• Update gitpod config files by @devlights in #32
• Update gitpod config files by @devlights in #33
• Update try-csharp.sln by @devlights in #34
• Fix-warnings by @devlights in #35
• Update csproj by @devlights in #36

Full Changelog: v1.4.0...v1.4.1