fix: resolve release notes GitHub tag format

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• centralize GitHub release tag and URL generation in CoreData
• fix WinUI release notes to open the resolved release page URL
• fix Avalonia release notes to try the resolved GitHub tag candidates before failing
• add regression tests for legacy and Devolutions calendar-version tag formats

Root cause

Devolutions-branded releases use GitHub tags prefixed with v such as v2026.1.2, while the app was building release note URLs from the raw version string such as 2026.1.2. That caused the Release notes action to open a GitHub 404 page and the Avalonia API lookup to fail.

Validation

• dotnet test src/UniGetUI.Core.Data.Tests/UniGetUI.Core.Data.Tests.csproj --verbosity minimal --nologo
• dotnet build src/UniGetUI/UniGetUI.csproj --nologo --verbosity minimal -p:Platform=x64 -p:RuntimeIdentifier=win-x64
• dotnet build src/UniGetUI.Avalonia/UniGetUI.Avalonia.csproj --nologo --verbosity minimal

Closes #4493