Patch vulnerability advisory (#3966)

Seems composer has a vulnerability, see GHSA-7c6p-848j-wh5h

Affected versions
>= 2.0.0-alpha1, < 2.2.23 -- patched in 2.2.23
>= 2.3.0-rc1, < 2.7.0 -- patched in 2.7.0

---

Let's raise the minimum to enforce the latest.

Thank you @PeopleInside for reporting this.

(cherry picked from commit e771b908d5e42ee223c6321ce264960d42b7ad9d)

composer.json

@@ -23,7 +23,7 @@
     },
     "require": {
         "flarum/core": "^1.8",
-        "composer/composer": "^2.3"
+        "composer/composer": "^2.7"
     },
     "require-dev": {
         "flarum/testing": "^1.0.0",