docs: correct SDK prerequisite to .NET 10.0+ in README (#35)

* docs: correct SDK prerequisite to .NET 10.0+ in README

The README listed '.NET 8.0+ SDK' as a prerequisite, but both
src/E4A.PostGuard.csproj and tests/E4A.PostGuard.Tests.csproj
multi-target net8.0;net10.0. Building the solution requires an SDK
that supports the highest target (net10.0); an 8.0 SDK cannot compile
the net10.0 target. CI installs both 8.0.x and 10.0.x SDKs accordingly.

Fixes #12

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs: drop justification parenthetical from SDK prerequisite

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: dobby-yivi-agent[bot] <275734547+dobby-yivi-agent[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: dobby-coder[bot]

README.md

@@ -33,7 +33,7 @@ See the [full API reference](https://docs.postguard.eu/repos/postguard-dotnet) f
 
 ### Prerequisites
 
-- .NET 8.0+ SDK
+- .NET 10.0+ SDK
 - Rust toolchain ([rustup](https://rustup.rs/))
 
 ### Build native library