feat: update to allow usage with PHP 8

- Adds `~8.0.0` to PHP constraint
- Makes minimum supported PHP version 7.3
- Updates PHPUnit version to 9.3, php-mock-phpunit to 2.0, and vfsStream to 1.6.7
  - Updates phpunit.xml.dist to 9.3 schema
  - Updates unit tests to work with PHPUnit 9.3
  - Removes all prophecy usage in favor of phpunit mock objects

Signed-off-by: Matthew Weier O'Phinney <matthew@weierophinney.net>

.gitignore

@@ -1,3 +1,4 @@
+/.phpunit.result.cache
 /clover.xml
 /coveralls-upload.json
 /phpunit.xml

composer.json

@@ -22,16 +22,16 @@
     "extra": {
     },
     "require": {
-        "php": "^5.6 || ^7.0",
+        "php": "^7.3 || ~8.0.0",
         "laminas/laminas-stdlib": "^3.1",
         "laminas/laminas-zendframework-bridge": "^1.0"
     },
     "require-dev": {
         "laminas/laminas-coding-standard": "~1.0.0",
-        "mikey179/vfsstream": "^1.6.4",
-        "mockery/mockery": "^1.0",
-        "php-mock/php-mock-phpunit": "^1.1.2 || ^2.0",
-        "phpunit/phpunit": "^5.7.27 || ^6.5.8 || ^7.1.5"
+        "mikey179/vfsstream": "^1.6.7",
+        "mockery/mockery": "^1.4.1",
+        "php-mock/php-mock-phpunit": "^2.0",
+        "phpunit/phpunit": "^9.3"
     },
     "autoload": {
         "psr-4": {