Update close button on secure spec configuration_form.rb

What does this MR do and why?

There was an update to gitlab-UI gitlab-ui!4505 (merged) which changed the close button to close-xs-button.

The Secret Push Protection toggle uses this, which caused a failure - https://gitlab.com/gitlab-org/gitlab/-/issues/480816

This MR updates it.

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Before	After

How to set up and validate locally

GITLAB_QA_USER_AGENT =<USER_AGENT> GITLAB_ADMIN_USERNAME=<ADMIN_USERNAME> GITLAB_ADMIN_PASSWORD="<ADMIN_PASSWORD>" GITLAB_USERNAME=<USERNAME> GITLAB_QA_ACCESS_TOKEN="<ACCESS_TOKEN>" GITLAB_PASSWORD="<GITLAB_PASSWORD>"  QA_DEBUG=true WEBDRIVER_HEADLESS=true bundle exec bin/qa Test::Instance::All https://staging.gitlab.com qa/specs/features/ee/browser_ui/13_secure/secret_push_protection_spec.rb