Support activerecord 8.1 explicitly

Author: ohbarye

.github/workflows/test.yml

@@ -26,13 +26,16 @@ jobs:
           - '7_1'
           - '7_2'
           - '8_0'
+          - '8_1'
           - 'latest'
         mysql-version:
           - '8.0'
         exclude:
           # Exclude conditions that don't meat the minimal requirement
           - ruby-version: '3.1'
             activerecord-version: '8_0'
+          - ruby-version: '3.1'
+            activerecord-version: '8_1'
 
           # Exclude duplicate conditions
           - ruby-version: '3.1'

gemfiles/activerecord_8_1.gemfile

@@ -0,0 +1,2 @@
+eval_gemfile("../Gemfile")
+gem "activerecord", "~> 8.1.0"