chore: reduce number of Dependabot PRs in-flight to 1

Part of #1590

Author: php-coder

.github/dependabot.yml

@@ -25,4 +25,4 @@ updates:
     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
+    open-pull-requests-limit: 1