data-model lib: Stop declaring compatibility with and testing at PHP <7.4

The library code seems to be using PHP 7.4+ only syntax,
so despite being allowed to be installed, the library wouldn't
work on older PHP versions.

Change-Id: Ic0f42f429e79c93b61330c9c3e2ecb671b6b0ef6

Author: manicki

.travis.yml

@@ -5,8 +5,6 @@ dist: xenial
 matrix:
   fast_finish: true
   include:
-    - php: 7.2
-    - php: 7.3
     - php: 7.4
     - php: 8.0
 

composer.json

@@ -27,7 +27,7 @@
 		"irc": "irc://irc.libera.chat/wikidata"
 	},
 	"require": {
-		"php": "^8.0|^7.1",
+		"php": "^8.0|^7.4",
 		"data-values/data-values": "^3.1.0",
 		"wikimedia/assert": "~0.2.2|~0.3.0|~0.4.0|~0.5.0"
 	},