Bump PHPCompatibility to current dev version (306cd263)

We were using version 9.3.5 that is from December 2019. Since
then lots of new sniffs and compatibility with PHP 8.x versions
has been provided.

So, while not ideal, let's use the dev version in the mean time.

Also, unrelated, add, amend a little bit the project keywords.

composer.json

@@ -3,9 +3,13 @@
     "type": "phpcodesniffer-standard",
     "description": "Moodle Coding Sniffer rules",
     "keywords": [
+        "moodle",
         "phpcs",
-        "standards",
-        "WordPress"
+        "coding",
+        "style",
+        "standard",
+        "sniff",
+        "codesniffer"
     ],
     "license": "GPL-2.0-or-later",
     "authors": [
@@ -22,7 +26,7 @@
         "dealerdirect/phpcodesniffer-composer-installer": "^1.0.0",
         "phpcsstandards/php_codesniffer": "^3.7.2",
         "phpcsstandards/phpcsextra": "^1.2.0",
-        "phpcompatibility/php-compatibility": "dev-develop#0a17f9ed"
+        "phpcompatibility/php-compatibility": "dev-develop#306cd263"
     },
     "config": {
         "allow-plugins": {