AP-5659: Freeze string literals and use unary + for mutable strings

Handle deprecation notices wanring of ruby 4.0 intention to default
string literals to be frozen/immutable.

```shell
/Users/joel.sugarman/github/ministryofjustice/laa-apply-for-legal-aid/app/helpers/hash_format_helper.rb:23:
  warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
```

app/helpers/hash_format_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module HashFormatHelper
   # These methods add classes to the HTML structure
   def format_hash(hash, html = "")
@@ -19,7 +21,7 @@ def format_hash(hash, html = "")
 
   def build_dl_array(key, value)
     content_tag(:dl, class: "govuk-body kvp govuk-!-margin-bottom-0") do
-      dl_contents = ""
+      dl_contents = +""
       dl_contents << content_tag(:dt, standardize_key(key))
       value.each do |val|
         dl_contents << if val.is_a?(Hash)
@@ -34,7 +36,7 @@ def build_dl_array(key, value)
 
   def build_dl(key, value)
     content_tag(:dl, class: "govuk-body kvp govuk-!-margin-bottom-0") do
-      dl_contents = ""
+      dl_contents = +""
       dl_contents << content_tag(:dt, standardize_key(key))
       dl_contents << if standard_type?(value)
                        content_tag(:dd, value.to_s.capitalize)