Add multi-framework support: netstandard2.1, net8.0, net10.0

[tonyqus]

Adds multi-targeting to the main library and upgrades the test/CI stack from the EOL netcoreapp6.0/.NET 6 baseline.

Changes

• ExcelFinancialFunctions.fsproj — TargetFramework → TargetFrameworks: netstandard2.0;netstandard2.1;net8.0;net10.0 (retains netstandard2.0 for backward compat)
• ExcelFinancialFunctions.Tests.fsproj — netcoreapp6.0 → net8.0
• CI workflows (pull-requests.yml, push-master.yml) — dotnet-version: 6.0.x → 8.0.x + 10.0.x; actions/checkout@v2 → @v4; actions/setup-dotnet@v1 → @v4
• common.fs — Fixes FS0041 overload ambiguity on DateTime introduced in .NET 8, which added a DateTime(DateOnly, TimeOnly, DateTimeKind) overload that confuses the F# compiler when argument types are unresolved:

// Before — ambiguous in .NET 8+
let date y m d = new DateTime(y, m, d)

// After
let date (y: int) (m: int) (d: int) = new DateTime(y, m, d)