github: workflows: stop testing 7.3

Author: kornrunner

.github/workflows/tests.yml

@@ -6,11 +6,11 @@ jobs:
 
   linux:
     name: Test on Linux
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
-        php-version: ['7.3', '7.4', '8.0']
+        php-version: ['7.4', '8.0', '8.1']
 
     steps:
       - uses: actions/checkout@v2
@@ -32,7 +32,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-version: ['7.3', '7.4', '8.0', '8.1']
+        php-version: ['7.4', '8.0', '8.1']
         arch: [x64]
         ts: [nts]