Pin devcontainer to Ruby 3.4 (#7825)

Author: lildude

.devcontainer/Dockerfile

@@ -1,3 +1,3 @@
 # Available versions: https://github.com/devcontainers/images/tree/main/src/ruby
-FROM mcr.microsoft.com/devcontainers/ruby
+FROM mcr.microsoft.com/devcontainers/ruby:3.4
 RUN apt update && apt install -y cmake