T135170 initially wanted to do this, but for some reason ended up doing c84ba4d8 instead to make purging more complex and fragile. Let's do it for real.
Timeline:
- 1.28: Update ApiPurge to emit a warning and log usage if used via GET
- 1.28: Communicate the deprecation to the mediawiki-api-announce mailing list
- 1.28/1.29: Review the usage statistics in Kibana, potentially alert high users directly
- 1.29: Announce removal date to the mediawiki-api-announce mailing list
- 1.29: Update ApiPurge to require POST
- 1.29: If possible, revert c84ba4d8.