No ETSI spec more recent than 2019-04

[tidoust]

Prompted by #790 and @daniel-montalvo, I looked into ETSI's data and the script created by @sandersaares to feed SpecRef. That script retrieves the catalog from ETSI's web site with an end date of 2019-04-16:
https://github.com/sandersaares/ScrapeEtsiDocuments/blob/master/ScrapeEtsiDocuments/Program.cs#L28

That seems to match the most recent publication date that exists in SpecRef for ETSI's data.