PHPStan: set the phpVersion

* Set the `phpVersion` to `70100` (lowest possible). This prevents PHPStan throwing notices which are related to the PHP version on which PHPStan is being _run_, in contrast to the PHP version which the code _supports_.
* With the `phpVersion` being set, PHPStan can now run on the `latest` PHP version in CI, though as long as the max PHPUnit version is PHPUnit 7.5, this _will_ throw deprecation notices, so let's leave that for now.

Includes minor update to the config for something which is no longer needed.

.github/workflows/phpstan.yml

@@ -19,7 +19,7 @@ concurrency:
 
 jobs:
   phpstan:
-    name: "PHP: 7.4 | PHPStan"
+    name: "PHPStan"
 
     runs-on: "ubuntu-latest"
 

phpstan.neon.dist

@@ -1,4 +1,5 @@
 parameters:
+    phpVersion: 70100 # Should be 50400, but PHPStan only accepts 70100 or higher...
     level: 0
     paths:
         - src
@@ -8,7 +9,7 @@ parameters:
         -
             count: 1
             message: '~^Undefined variable: \$phpCodeSnifferConfig$~'
-            path: %currentWorkingDirectory%/src/Config.php
+            path: src/Config.php
 
     dynamicConstantNames:
         - PHP_CODESNIFFER_IN_TESTS