You need to install .NET 7 and the language server manually:
# uncomment the --prerelease flag if you want to use the latest preview version
dotnet tool install -g rcaron.languageserver # --prerelease
You need to have your global .NET tools directory in your PATH environment variable, which is in most cases ~/.dotnet/tools.
And then you can use the language server from RCaron, because the rcaron.languageServerPath setting is set to rcaron-languageserver by default.