chore(dotnet): use NuGet lock file (`packages.lock.json`)

[Flash0ver]

See SDK-1314

[linear-code]

NET-700

[Flash0ver]

This might be less straight forward than I thought:
Although lock-files do work when multi-targeting TFMs, we are running issues in CI/CD across platforms:
E.g. macOS-generated lockfiles will cause TFM mismatch errors on Linux

We have platform-conditional TFMs (iOS/macCatalyst only on macOS, Android on all), so a single lockfile can't be used with --locked-mode across all CI platforms.

We would need to use per-platform lock files and have separate restore steps per platform using --lock-file-path.

See also

• https://devblogs.microsoft.com/dotnet/enable-repeatable-package-restores-using-a-lock-file/
• https://github.com/NuGet/Home/wiki/Enable-repeatable-package-restore-using-lock-file
• https://learn.microsoft.com/nuget/consume-packages/package-references-in-project-files

[jamescrosswell]

@Flash0ver given that we always specify a version for NuGet dependencies, would adding a NuGet lock file make any difference or is it redundant?