MDL-78993 theme: Remove $CFG->svgicons

The $CFG->svgicons setting was introduced in Moodle 2.4 due to incomplete
SVG support in certain web browsers.
The landscape has evolved significantly since then, and all modern browsers
now handle SVG files correctly.
The $CFG->svgicons settings has been removed and the supports_svg() method
has been updated with currently supported browsers (IE support was removed
in Moodle 3.10).

message/tests/api_test.php

@@ -1760,7 +1760,7 @@ class api_test extends messagelib_test {
         // Consider first conversations is self-conversation.
         $this->assertEquals(2, $conversations[0]->membercount);
         $this->assertEquals($course1->shortname, $conversations[0]->subname);
-        $this->assertEquals('https://www.example.com/moodle/theme/image.php/_s/boost/core/1/g/g1', $conversations[0]->imageurl);
+        $this->assertEquals('https://www.example.com/moodle/theme/image.php/boost/core/1/g/g1', $conversations[0]->imageurl);
 
         // Now, disable the conversation linked to the group and verify it's no longer returned.
         $DB->set_field('message_conversations', 'enabled', 0, ['id' => $conversations[0]->id]);