T261872: Drop PHP 7.2 support from MediaWiki master branch, once Wikimedia production is on 7.3 handles the same topic for core.
Once this is done wikibase can no longer operate on 7.2 systems (as it itself depends on MediaWiki core).
When should then
- stop testing wikibase against 7.2
- these ideas were already applied to the REL1_35 branch, which is an LTS release, to prevent having to support PHP 7.2 in the long run. Wikibase patch: https://gerrit.wikimedia.org/r/630874/