test: use a supported PHP version

Author: Greg Bowler

.github/workflows/ci-php71.yml .github/workflows/ci-php73.yml.github/workflows/ci-php71.yml renamed to .github/workflows/ci-php73.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - uses: php-actions/composer@v6
 
@@ -19,4 +19,4 @@ jobs:
           bootstrap: vendor/autoload.php
           configuration: test/phpunit.xml
           args: --coverage-text
-          php_version: "7.1"
+          php_version: "7.3"