UpdateConfigBackend: use api4 to flush the cache

[mlutfy]

Overview

CiviCRM has a few places where it flushes the cache, and sometimes they work in slightly different ways.

This removes one of those slightly different ways.

Before

Flushing the cache would not refresh the list of installed reports provided by an extension.

(It's a custom extension where new mgd files are being added by devs)

Running system.flush via the API worked fine, but not when using this form/button.

After

Flushing the cache works as expected.

[civibot]

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[mattwire]

test fail unrelated