Add php 8.1 to the build pipeline matrix

Author: xico42

.github/workflows/checks.yml

@@ -33,6 +33,10 @@ jobs:
           -
             version: 8.0
             xdebug: 3.1.1
+
+          -
+            version: 8.1.0RC3
+            xdebug: 3.1.1
     runs-on: ubuntu-20.04
     steps:
       -
@@ -81,12 +85,18 @@ jobs:
           -
             version: 8.0
             composer: --prefer-lowest
+          -
+            version: 8.1.0RC3
+            composer: --prefer-lowest
           -
             version: 7.4
             composer: --prefer-stable
           -
             version: 8.0
             composer: --prefer-stable
+          -
+            version: 8.1.0RC3
+            composer: --prefer-stable
     steps:
       -
         name: Download sources