[core] Drop php 5.6 and php 7.0 support (#2224)

composer.json

@@ -21,7 +21,7 @@
         "rss": "https://github.com/RSS-Bridge/rss-bridge/commits/master.atom"
     },
     "require": {
-        "php": ">=5.6",
+        "php": ">=7.1",
         "ext-mbstring": "*",
         "ext-curl": "*",
         "ext-openssl": "*",
@@ -30,7 +30,7 @@
         "ext-json": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6 || ^7"
+        "phpunit/phpunit": "^7"
     },
     "suggest": {
         "ext-memcached": "Allows to use memcached as cache type",