Check up and remove require of old version libraries

[reedy]

For example, https://github.com/wmde/WikibaseDataModelServices/blob/master/composer.json#L28 has

"wikibase/data-model": "~9.0|~8.0|~7.0|~6.3",

We're not testing on 6, so should we really be allowing said version? Similar questions maybe for data-values/data-value and wikimedia/assert

	"require": {
		"php": ">=7.0",
		"wikibase/data-model": "~9.0|~8.0|~7.0|~6.3",
		"data-values/data-values": "~2.0|~1.0",
		"diff/diff": "~3.2",
		"wikimedia/assert": "~0.2.2|~0.3.0|~0.4.0"
	},