update rbconfig spec to Unicode version 15.0.0

(this includes emoji version 15.0)

spec/ruby/library/rbconfig/unicode_emoji_version_spec.rb

@@ -16,8 +16,8 @@ describe "RbConfig::CONFIG['UNICODE_EMOJI_VERSION']" do
 
   # Caution: ruby_version_is means is_or_later
   ruby_version_is "3.2" do
-    it "is 14.0" do
-      RbConfig::CONFIG['UNICODE_EMOJI_VERSION'].should == "14.0"
+    it "is 15.0" do
+      RbConfig::CONFIG['UNICODE_EMOJI_VERSION'].should == "15.0"
     end
   end
 end

spec/ruby/library/rbconfig/unicode_version_spec.rb

@@ -16,8 +16,8 @@ describe "RbConfig::CONFIG['UNICODE_VERSION']" do
 
   # Caution: ruby_version_is means is_or_later
   ruby_version_is "3.2" do
-    it "is 14.0.0" do
-      RbConfig::CONFIG['UNICODE_VERSION'].should == "14.0.0"
+    it "is 15.0.0" do
+      RbConfig::CONFIG['UNICODE_VERSION'].should == "15.0.0"
     end
   end
 end