adjust constraint versions

Author: hmdros

.github/workflows/main.yml

@@ -21,10 +21,9 @@ jobs:
         run: |
           RUBY_VERSION_STR="${{ matrix.ruby-version }}"
           case "$RUBY_VERSION_STR" in
-            2.6*) BUNDLER_CONSTRAINT=">= 2, < 3" ;;
-            2.7*) BUNDLER_CONSTRAINT=">= 2, < 4" ;;
-            3.*)  BUNDLER_CONSTRAINT=">= 3, < 4" ;;
-            *)    BUNDLER_CONSTRAINT=">= 2" ;;
+            2.6*) BUNDLER_CONSTRAINT=">= 2.4.0, < 2.7.0" ;;
+            2.7*) BUNDLER_CONSTRAINT=">= 2.4.0, < 2.7.0" ;;
+            3.*)  BUNDLER_CONSTRAINT=">= 2.7.0, < 4" ;;
           esac
 
           gem install bundler -v "$BUNDLER_CONSTRAINT"