Update PHPUnit to 8.x (#11635)

* Shift return type of base TestCase methods

From the [PHPUnit 8 release notes][1], the `TestCase` methods below now declare a `void` return type:

- `setUpBeforeClass()`
- `setUp()`
- `assertPreConditions()`
- `assertPostConditions()`
- `tearDown()`
- `tearDownAfterClass()`
- `onNotSuccessfulTest()`

[1]: https://phpunit.de/announcements/phpunit-8.html

* Update PHPUnit to 8.x

Part of Laravel 6 upgrade

* Bump php versions

Co-authored-by: Laravel Shift <[email protected]>
Co-authored-by: Tony Murray <[email protected]>

tests/Unit/Data/PrometheusStoreTest.php

@@ -41,7 +41,7 @@ use LibreNMS\Tests\TestCase;
  */
 class PrometheusStoreTest extends TestCase
 {
-    public function setUp() : void
+    protected function setUp() : void
     {
         parent::setUp();