ci(bindings/dotnet): drive releases via workflow_dispatch with previe…

[Fatorin]

Rationale for this change

The initial release_dotnet.yml used a shared v* tag trigger, which would fire on core and other bindings' tags as well.
Since NuGet packages cannot be deleted (only unlisted) and NuGet has no staging repository, any accidental publish is irreversible.

After discussion with a maintainer, releases are driven entirely via workflow_dispatch — every publish is an explicit, intentional action.

What changes are included in this PR?

• Remove the push: tags: trigger.
• Replace the dotnet_publish boolean with a release_type choice input (none / preview / rc).
• Switch .csproj from <Version> to <VersionPrefix> and use dotnet pack --version-suffix to assemble the final version.

release_type	Version	Publishes to
none (default)	{VersionPrefix}-preview.{run_number}	GitHub Actions artifact only (dry run)
preview	{VersionPrefix}-preview.{run_number}	nuget.org
rc	{VersionPrefix}-rc	nuget.org

Are there any user-facing changes?

No.

AI Usage Statement

This PR was developed with assistance from Claude (Anthropic) for research and drafting.