build: bump shipping libraries from .NET Framework 4.5.2 to 4.7.2 (consolidate target frameworks step 2)

[saveenr]

Bump the shipping libraries (VisioAutomation, VisioAutomation.Models, VisioScripting, VisioPowerShell) and sample projects (VSamples, VSamples.Docs) from .NET Framework 4.5.2 to 4.7.2. Test projects are already on 4.7.2; this consolidates the whole solution onto a single TFM.

Why

Two reasons:

1. VS 2026 floor. VS 2026 supports .NET Framework 4.6.2 and above only. The 4.5.2 floor blocks Move development environment to Visual Studio 2026 #171 (move dev to VS 2026).
2. TFM consistency. Mixed TFMs cause subtle binary-compatibility surprises (a test project on a higher TFM can use APIs the library under test cannot). Step 1 (4.0/4.5 → 4.5.2) eliminated the production split; this step eliminates the 4.5.2/4.7.2 library/test split.

Constraint

Do not land before 2026-10-13 (Windows 10 LTSB 2016 leaves Extended Support). Bumping earlier blocks enterprise users on locked LTSB images. See enterprise_compat_ltsb2016.md in the project memory.

VisioPowerShell older-PowerShell support is preserved

The older-PS floor is set by the System.Management.Automation v3 reference and the manifest's PowerShellVersion value, not by the .NET Framework TFM. Bumping 4.5.2 → 4.7.2 doesn't change that.

Cross-refs

• Move development environment to Visual Studio 2026 #171 (Move dev to VS 2026), drives this.
• docs/futures/build-and-code.md, entry "Consolidate target frameworks".

Effort

S. The per-csproj change is mechanical. Verify nothing in the test suite breaks under the new TFM.