Pull Request for Issue #43505 .

Summary of Changes

When an extension provides a changelog URL in its update XML, a button is displayed to show the changelog, but the URL only loads the changelog from the current extension, not from the update site.

Testing Instructions

1. Install an old version of a third party extension, for example a language pack.
2. Search for an update for that extension.
3. We now try to emulate a changelog available for that extension: Go to the database and in #__updates look for the entry for your extension update. Edit the changelogurl column and insert a URL to a changelog.xml in your sites root, for example http://localhost/changelog.xml
4. Create a changelog.xml with the content below. (Adapt the content to the extension you've been using for this)
5. Go to the "update extensions" view and see the button for the changelog for your extension.

Content for the changelog.xml

<changelogs>
	<changelog>
		<element>pkg_ru-RU</element>
		<type>package</type>
		<version>5.2.2.1</version>
		<security>
			<item>Item A</item>
			<item><![CDATA[<h2>You MUST replace this file</h2>]]></item>
		</security>
		<fix>
			<item>Item A</item>
			<item>Item b</item>
		</fix>
	</changelog>
</changelogs>

Actual result BEFORE applying this Pull Request

A modal opens and is empty.

Expected result AFTER applying this Pull Request

A modal opens and shows a few entries

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed