Optimize string transformation using String#tr

This change was flagged by RuboCop and has been verified for
correctness and performance improvement. While Ruby 2.1 is no longer
tested in CI, `String#tr` is supported since that version, ensuring
backwards compatibility.

- Replace `gsub(/_/, '-')` with `tr('_', '-')` for better performance
- Improve `bench benchmark/data_attribute.haml` execution time by 20%
- Maintain compatibility with Ruby 2.1 (Haml's minimum required version)

Before:
```
### IPS
         haml v6.3.0     94.400k i/s (0.011ms) -    473.668k

### Memory
         haml v6.3.0     3.264k memsize (   600.000  retained)
                        41.000  objects (     9.000  retained)
                        19.000  strings (     6.000  retained)
```

After:
```
### IPS
         haml v6.3.0    113.201k i/s (0.009ms) -    568.672k

### Memory
         haml v6.3.0     2.808k memsize (   360.000  retained)
                        37.000  objects (     6.000  retained)
                        19.000  strings (     5.000  retained)
```

Refs:
- https://docs.ruby-lang.org/en/2.1.0/String.html#method-i-tr
- https://docs.rubocop.org/rubocop-performance/cops_performance.html#performancestringreplacement

lib/haml/attribute_builder.rb

@@ -105,7 +105,7 @@ def flatten_attributes(attributes)
             if k.nil?
               flattened[key] = v
             else
-              flattened["#{key}-#{k.to_s.gsub(/_/, '-')}"] = v
+              flattened["#{key}-#{k.to_s.tr('_', '-')}"] = v
             end
           end
         else