chore: configure Dependabot to have no more than 2 PRs in-flight

Part of #1590

Author: php-coder

.github/dependabot.yml

@@ -24,3 +24,5 @@ updates:
     assignees: [ "php-coder" ]
     reviewers: [ "php-coder" ]
     labels: [ "kind/dependency-update" ]
+    # https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#open-pull-requests-limit
+    open-pull-requests-limit: 2