Target net10.0 only and centralize the TFM

[perkops]

Summary

• Drop net8.0; target net10.0 exclusively
• Define the TFM once in the root Directory.Build.props
• Remove now-redundant per-project TFM declarations

Changes

🔧 Configuration

• Set net10.0 in root props
• Remove from Atc.Test.csproj
• Remove redundant from the test project

🔥 Removal

• Drop net8.0 from the supported target frameworks

📝 Documentation

• Remove the single-TFM references from the README

Breaking Changes

• The package now targets net10.0 only.
• Consumers on net8.0/net9.0 can no longer restore Atc.Test;
  they must move to net10.0.

Notes

• TFM is now inherited solution-wide from Directory.Build.props,
  so new projects need no per-project target declaration.

BEGIN_COMMIT_OVERRIDE
feat!: drop netstandard2.1/net8.0/net9.0; target net10.0 exclusively
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