This migrates the CentOS guest plugin to identify machines by what is in os-release rather than based on if the /etc/centos-release file exists. It just so happens that the Rocky Linux plugin is set up to inherit directly from :redhat (not :centos) and thus the CentOS flavor is attempted before Rocky in the depth first search. For some reason the rockylinux/8 vagrant box still has the /etc/centos-release file in it even though it's only supposed to inherit from RHEL. The almalinux/9 box does not have /etc/centos-release.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.
Generate proto
grpc_tools_ruby_protoc -I . --ruby_out=gen/plugin --grpc_out=gen/plugin ./plugin_server.proto