docs: refresh README for .NET 10 migration

[phmatray]

Summary

The repository has been migrated to .NET 10 (SDK 10.0.103, TFM net10.0, C# 14) but the README still references .NET 8 throughout.

Changes Needed

• Update tagline from .NET 8 to .NET 10
• Update .NET badge from 8.0 to 10.0 with correct download URL
• Update Tech Stack table: .NET 8.0 / C# 12 → .NET 10.0 / C# 14
• Update Blazor SSR version reference: 8.0 → 10.0
• Update FastEndpoints version: 5.21 → 8.0
• Update prerequisites: .NET SDK >= 8.0 → .NET SDK >= 10.0
• Update license copyright year: 2020-2024 → 2020-2026
• Update any other stale version references

Context

• global.json: SDK 10.0.103
• Directory.Build.props: <TargetFramework>net10.0</TargetFramework>, <LangVersion>14</LangVersion>
• FastComponents.csproj: FastEndpoints 8.0.1, AngleSharp 1.4.0, AspNetCore Components 10.0.3