chore: drop support for php 7.3

Author: imorland

.github/workflows/backend.yml

@@ -4,9 +4,10 @@ on: [workflow_dispatch, push, pull_request]
 
 jobs:
   run:
-    uses: flarum/framework/.github/workflows/REUSABLE_backend.yml@main
+    uses: flarum/framework/.github/workflows/REUSABLE_backend.yml@1.x
     with:
       enable_backend_testing: false
       enable_phpstan: true
+      php_versions: '["7.4", "8.0", "8.1", "8.2", "8.3"]'
 
       backend_directory: .

.github/workflows/frontend.yml

@@ -4,7 +4,7 @@ on: [workflow_dispatch, push, pull_request]
 
 jobs:
   run:
-    uses: flarum/framework/.github/workflows/REUSABLE_frontend.yml@main
+    uses: flarum/framework/.github/workflows/REUSABLE_frontend.yml@1.x
     with:
       enable_bundlewatch: false
       enable_prettier: true

composer.json

@@ -36,6 +36,7 @@
         }
     ],
     "require": {
+        "php": "^7.4 || ^8.0",
         "flarum/core": "^1.7.0"
     },
     "replace": {