qual: pin phpunit 9.6.19 in order to keep supporting PHP 7

Author: lourot

.github/dependabot.yml

@@ -38,9 +38,8 @@ updates:
     commit-message:
       prefix: 'chore: '
     ignore:
-      # Staying on phpunit < 11 in order to keep supporting PHP 7.
+      # Staying on phpunit 9.6.19 in order to keep supporting PHP 7.
       - dependency-name: 'phpunit/phpunit'
-        update-types: ['version-update:semver-major']
 
     # Do not get one pull request per version bump. Instead get one pull request
     # bumping several dependency versions at once: