adds an option to disable the setting of legacy headers

[therephil]

As per MDN and OWASP it is recommended to not use deprecated security headers.
This PR enables passing that parameter to paragonie/csp-builder

[what-the-diff]

PR Summary

• Introduction of 'Legacy' Option in Security Settings
  A new option named 'legacy' has been added in the security settings. This will help in managing the older and potentially deprecated security header generation.

• Updation in the 'sendHeaders' method
  The 'sendHeaders' procedure has been enhanced to accept the 'legacy' option. This will influence the method of sending security header.

• Default Setting of 'Legacy' Option
  The 'legacy' option is set to 'true' as the default setting. This means old security header generation is enabled automatically.

• Readme Enhancement for 'Legacy headers'
  The readme file has been updated with a new section explaining 'Legacy headers'. This provides information on potential issues linked with deprecated headers and how they can be used.

• Inclusion of 'Legacy' Option in Settings Table
  The new 'legacy' option has been included in the settings table detailed in the readme file. This allows users to understand its functionality better.