Add Ruby 3.4 to CI matrix and make it default

Add the newly released Ruby 3.4 to the CI build matrix and make it the
default version for all non-test jobs.

Author: brandur

.github/workflows/ci.yaml

@@ -14,7 +14,7 @@ env:
   #
   # If updating this value, you probably also want to add a new version to the
   # spec version matrix below.
-  RUBY_VERSION: "3.3"
+  RUBY_VERSION: "3.4"
 
   # A suitable URL for a test database.
   TEST_DATABASE_NAME: river_test
@@ -134,6 +134,7 @@ jobs:
           - "3.1"
           - "3.2"
           - "3.3"
+          - "3.4"
 
     services:
       postgres: