EPPlus 9: centralize versioning, embed sub-project DLLs, drop net35

[swmal]

Summary

Prepares EPPlus 9 for its multi-assembly package structure, where four
previously separate projects ship as embedded DLLs inside the main EPPlus
package.

Changes

Versioning

• Centralized Version, AssemblyVersion and FileVersion (9.0.1) in
  src/Directory.Build.props, shared by EPPlus and the four embedded
  sub-projects (DrawingRenderer, Export.Pdf, Fonts.OpenType, Graphics).
• EPPlus.Interfaces and EPPlus.System.Drawing keep their own independent
  versions.
• Added a shared EPPlusCoreTargetFrameworks property in
  src/Directory.Build.props, consumed explicitly per project via
  <TargetFrameworks>$(EPPlusCoreTargetFrameworks)</TargetFrameworks>. It is
  not applied automatically, so the separately versioned packages and the
  test/benchmark projects are unaffected.

Packaging

• Sub-project DLLs are embedded in the EPPlus package via
  IncludeReferencedProjectsInPackage; set IsPackable=false on the four
  sub-projects and the benchmark project so they no longer produce standalone
  packages.
• PrivateAssets="all" on the four ProjectReference entries, so the
  sub-projects are not emitted as NuGet dependencies of EPPlus — they ship as
  embedded DLLs, not as packages consumers resolve separately.

Target frameworks

• Dropped net35; removed its build config across the csproj files.
• Aligned net10.0 dependencies with net9.0 in the sub-projects.

Build/Release workflow

• Removed the standalone dotnet pack step (produced an incomplete package);
  the package is now produced by the build and collected from bin/Release.
• Added a step verifying all five embedded DLLs are present and validly signed.

Notes

PackageReleaseNotes will be updated before the v9 preview publication.