Be explicit about the supported versions of PHP

The constraint used for the PHP version is far too permissive as we
cannot predict if the library will be compatible with future versions of
PHP without any change.

Author: LeSuisse

composer.json

@@ -3,7 +3,7 @@
     "description": "Prometheus instrumentation library",
     "license": "Apache-2.0",
     "require": {
-        "php": ">=7.3",
+        "php": "^7.3|^7.4|^8.0",
         "ext-json": "*",
         "psr/http-client": "^1.0",
         "psr/http-factory": "^1.0",