Longer MTA STS max age.

Currently you can only select up to 7 days as mage age on the policy, as per the UK NCSC recommendation to have minimum 2 weeks when on enforce can we have the option. https://www.ncsc.gov.uk/collection/email-security-and-anti-spoofing/using-mta-sts-to-protect-the-privacy-of-your-emails