Update README.md for Ruby 3.1+ requirement

Author: cllns

README.md

@@ -14,7 +14,7 @@
 
 This library officially supports the following Ruby versions:
 
-* MRI `>= 2.4.0`
+* MRI `>= 3.1.0`
 * jruby `>= 9.4` (not tested on CI)
 
 ## License