[TASK] Update phpunit/phpunit requirement from ^5.7.27 to ^8.5.36 (#424)

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/8.5.36/ChangeLog-8.5.md)
- [Commits](sebastianbergmann/phpunit@5.7.27...8.5.36)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

composer.json

@@ -19,7 +19,7 @@
         "ext-iconv": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7.27",
+        "phpunit/phpunit": "^8.5.36",
         "codacy/coverage": "^1.4.3"
     },
     "suggest": {

phpunit.xml

@@ -1,6 +1,6 @@
 <phpunit
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/5.7/phpunit.xsd">
+    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/8.5/phpunit.xsd">
     <testsuites>
         <testsuite name="Project Test Suite">
             <directory>tests</directory>