restore unicode_normalize_kc as a deprecated method (#504)

As discussed in #492 (comment), this change restores `unicode_normalize_kc` as a deprecated method (in case some people where using it). Example of the produced warning:
```
NOTE: Addressable::IDNA.unicode_normalize_kc is deprecated; use String#unicode_normalize(:nfkc) instead. It will be removed on or after 2023-04.
Addressable::IDNA.unicode_normalize_kc called from benchmark/unicode_normalize.rb:17.
```

benchmark/unicode_normalize.rb

@@ -2,7 +2,7 @@
 # frozen_string_literal: true.
 
 require "benchmark"
-require "addressable/idna/pure.rb"
+require_relative "../lib/addressable/idna/pure.rb"
 require "idn"
 
 value = "fiﾯリ宠퐱卄.com"

lib/addressable/idna/native.rb

@@ -29,6 +29,16 @@ def self.punycode_decode(value)
        IDN::Punycode.decode(value.to_s)
      end
 
+    class << self
+      # @deprecated Use {String#unicode_normalize(:nfkc)} instead
+      def unicode_normalize_kc(value)
+        value.to_s.unicode_normalize(:nfkc)
+      end
+
+      extend Gem::Deprecate
+      deprecate :unicode_normalize_kc, "String#unicode_normalize(:nfkc)", 2023, 4
+    end
+
     def self.to_ascii(value)
       value.to_s.split('.', -1).map do |segment|
         if segment.size > 0 && segment.size < 64

lib/addressable/idna/pure.rb

@@ -112,6 +112,16 @@ def self.to_unicode(input)
       output
     end
 
+    class << self
+      # @deprecated Use {String#unicode_normalize(:nfkc)} instead
+      def unicode_normalize_kc(value)
+        value.to_s.unicode_normalize(:nfkc)
+      end
+
+      extend Gem::Deprecate
+      deprecate :unicode_normalize_kc, "String#unicode_normalize(:nfkc)", 2023, 4
+    end
+
     ##
     # Unicode aware downcase method.
     #