Remove deprecated calls to String#mb_chars in ActiveRecord

Related to  rails#54081
cc @byroot

When adding an index like the following :
```ruby
add_reference :posts, :user, foreign_key: { on_delete: :nullify }, index: true, null: true
```

This triggers a deprecation warning. The fix is to use
`String#truncate_bytes` from ActiveSupport to properly limit the index
name size.

Author: chaadow

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -1605,7 +1605,7 @@ def generate_index_name(table_name, column)
           name = "idx_on_#{Array(column) * '_'}"
 
           short_limit = max_index_name_size - hashed_identifier.bytesize
-          short_name = name.mb_chars.limit(short_limit).to_s
+          short_name = name.truncate_bytes(short_limit, omission: nil)
 
           "#{short_name}#{hashed_identifier}"
         end