Improve performance by avoiding regex construction (#20215)

```ruby
10.times { p /#{FOO}/.object_id }
10.times { p FOO_RE.object_id }
```

app/lib/hashtag_normalizer.rb

@@ -8,7 +8,7 @@ class HashtagNormalizer
   private
 
   def remove_invalid_characters(str)
-    str.gsub(/[^[:alnum:]#{Tag::HASHTAG_SEPARATORS}]/, '')
+    str.gsub(Tag::HASHTAG_INVALID_CHARS_RE, '')
   end
 
   def ascii_folding(str)