Migrate to centralized NuGet package management (CPM)

[standleypg]

Introduces Directory.Packages.props to centralize NuGet package version management across all projects in the solution. All package versions are now defined in a single location.

Changes

• Added Directory.Packages.props at the solution root.
• Removed Version attributes from all <PackageReference> elements in .csproj files.
• Consolidated package version definitions into the centralized file.

Notes

This change does not affect runtime behavior. It standardizes dependency management and simplifies future version updates.

[Copilot]

Pull request overview

This pull request migrates the YoutubeExplode solution to use centralized NuGet package management (CPM) by introducing a Directory.Packages.props file at the solution root. This standardizes how package versions are managed across all projects, making it easier to maintain consistent dependency versions and simplify future updates.

Changes:

• Added Directory.Packages.props with centralized version definitions for all 24 NuGet packages used across the solution
• Removed Version attributes from all <PackageReference> elements in 6 project files while preserving other attributes like PrivateAssets and Condition
• Unified the PolyShim package version from 2.3.0 (in Converter project) to 2.5.0 (matching other projects)

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Directory.Packages.props	New file defining centralized package versions for all 24 NuGet packages with ManagePackageVersionsCentrally enabled
YoutubeExplode/YoutubeExplode.csproj	Removed version attributes from 9 package references while maintaining conditional references and other attributes
YoutubeExplode.Tests/YoutubeExplode.Tests.csproj	Removed version attributes from 8 test-related package references
YoutubeExplode.Demo.Gui/YoutubeExplode.Demo.Gui.csproj	Removed version attributes from 7 Avalonia and UI-related package references
YoutubeExplode.Demo.Cli/YoutubeExplode.Demo.Cli.csproj	Removed version attribute from single CSharpier.MsBuild package reference
YoutubeExplode.Converter/YoutubeExplode.Converter.csproj	Removed version attributes from 4 package references, now using unified PolyShim version
YoutubeExplode.Converter.Tests/YoutubeExplode.Converter.Tests.csproj	Removed version attributes from 8 test-related package references

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[Tyrrrz]

Thank you for the PR!