Revert back to 3.0 compatible syntax. Add 3.0 to CI.

Author: simi

.github/workflows/build.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["3.1", "3.2", "3.3", "3.4", "jruby"]
+        ruby: ["3.0", "3.1", "3.2", "3.3", "3.4", "jruby"]
 
     steps:
       - name: Checkout code

.rubocop.yml

@@ -4,7 +4,7 @@ plugins:
 
 AllCops:
   DisabledByDefault: false
-  TargetRubyVersion: 3.1
+  TargetRubyVersion: 3.0
   NewCops: enable
   SuggestExtensions: false
   Exclude:

Gemfile.lock

@@ -115,4 +115,4 @@ DEPENDENCIES
   yard (>= 0.5.8)
 
 BUNDLED WITH
-   2.6.8
+   2.5.23

lib/xpath/union.rb

@@ -15,8 +15,8 @@ def expression
       :union
     end
 
-    def each(&)
-      arguments.each(&)
+    def each(&block)
+      arguments.each(&block)
     end
 
     def method_missing(*args) # rubocop:disable Style/MissingRespondToMissing