Remove the old expiry date information for ‘Send files by email’

[karlchillmaid]

The ‘Send files by email’ section of our API documentation says:

Files sent before 12 April 2023 had a longer default period of 78 weeks (18 months).

We’re now well past this, all those files should have expired, so we can remove this line as it’s irrelevant.

[klssmith]

If you search the file schema.json there's also mention of the old expiry date there (it's a relatively new file)