Add Ruby 4.0 to CI configuration and update README for supported versions (#469)

Author: ydah

.github/workflows/ci.yaml

@@ -17,6 +17,7 @@ jobs:
           - "3.2"
           - "3.3"
           - "3.4"
+          - "4.0"
           - ruby-head
     runs-on: ubuntu-latest
     steps:

README.md

@@ -12,6 +12,7 @@ Committee is tested on the following MRI versions:
 - 3.2
 - 3.3
 - 3.4
+- 4.0
 
 ## Committee::Middleware::RequestValidation