Rubocop fix: Perfomance/UnfreezeString (#26217)

app/views/well_known/host_meta/show.xml.ruby

@@ -2,6 +2,13 @@
 
 doc = Ox::Document.new(version: '1.0')
 
+ins = Ox::Instruct.new(:xml).tap do |instruct|
+  instruct[:version] = '1.0'
+  instruct[:encoding] = 'UTF-8'
+end
+
+doc << ins
+
 doc << Ox::Element.new('XRD').tap do |xrd|
   xrd['xmlns'] = 'http://docs.oasis-open.org/ns/xri/xrd-1.0'
 
@@ -11,4 +18,4 @@ doc << Ox::Element.new('XRD').tap do |xrd|
   end
 end
 
-"<?xml version=\"1.0\" encoding=\"UTF-8\"?>#{Ox.dump(doc, effort: :tolerant)}".force_encoding('UTF-8')
+Ox.dump(doc, effort: :tolerant).force_encoding('UTF-8')