Modernize builds: add GitHub Actions CI, SDK-style projects, and extended .NET Framework targets

[forcewake]

Motivation

• Incrementally modernize the repository to enable usage from modern .NET runtimes while preserving legacy .NET Framework compatibility.
• Provide reproducible CI coverage across the supported .NET Framework aliases and add a path to build SDK-style projects for .NET Core/.NET 8.0.
• Document the intended modernization roadmap and migration strategy.

Description

• Add a GitHub Actions workflow at .github/workflows/ci.yml that restores and builds the solution across a Windows matrix of framework aliases using nuget restore and msbuild, and builds SDK-style projects on ubuntu-latest using dotnet build.
• Introduce SDK-style projects under src/*/*.Modern (FlatFile.Core.Modern, FlatFile.Core.Attributes.Modern, FlatFile.Delimited.Modern, FlatFile.FixedLength.Modern, FlatFile.Delimited.Attributes.Modern, FlatFile.FixedLength.Attributes.Modern) targeting netstandard2.0;net8.0 and compiling the existing source files.
• Extend many legacy project files and the psake build script (assets/psake-common.ps1) to include additional framework aliases (NET451, NET452, NET46, NET461, NET462, NET47, NET471, NET472, NET48) to enable multi-target builds for .NET Framework consumers.
• Add docs/modernization-plan.md, update README.md with modernization status, and add a small compatibility-related change to ParseLineException (serialization constructor wrapped with #pragma warning disable/restore SYSLIB0051).

Testing

• No automated test suite was executed as part of this change; the contribution adds a CI workflow to run builds but the workflow has not been executed within this PR.
• The intended automated build steps for CI are the nuget restore + msbuild matrix for legacy targets and dotnet build for the new SDK-style projects as defined in .github/workflows/ci.yml.

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Codex Task