chore: Upgrade rails and ruby versions (#2400)

ruby version: 3.0.2
rails version: 6.1.4

.devcontainer/Dockerfile.base

@@ -1,5 +1,5 @@
 # pre-build stage
-ARG VARIANT=2.7
+ARG VARIANT=3
 FROM mcr.microsoft.com/vscode/devcontainers/ruby:${VARIANT}
 
 # Update args in docker-compose.yaml to set the UID/GID of the "vscode" user.

.devcontainer/docker-compose.yml

@@ -10,8 +10,8 @@ services:
       context: ..
       dockerfile: .devcontainer/Dockerfile
       args:
-        # Update 'VARIANT' to pick a Ruby version: 2, 2.7, 2.6, 2.5
-        VARIANT: 2.7
+        # Update 'VARIANT' to pick a Ruby version: https://github.com/microsoft/vscode-dev-containers/tree/main/containers/ruby
+        VARIANT: 3
         # [Choice] Install Node.js
         INSTALL_NODE: "true"
         NODE_VERSION: "lts/*"