v4.0.0: reduce dependencies, bump major Serilog versions, update target frameworks

What's Changed

• build: reduce dependencies, by @tboby #75
• build: bump major versions of Serilog deps, by @colin-anderson #76
• build: update target frameworks, to match official Serilog repos, by @colin-anderson #76

Full Changelog: v3.0.2...v4.0.0