Use original string hook callback definition for backwards compat.

[paulholden]

See #809 - in order to support this hook from Moodle 4.3 we should use the simple callback format. Support for array callable notation arrived only in 4.4 (MDL-81180).

[paulholden]

To clarify, this is only the hook notation that needs to be updated, because when Moodle 4.3 attempts to read this hook definition in Moodle 4.3 the following is emitted:

debugging() message/s found: Hook callback definition contains invalid 'callback' string in 'auth_saml2'
line 536 of /lib/classes/hook/manager.php: call to debugging()
line 501 of /lib/classes/hook/manager.php: call to core\hook\manager->normalise_callback()
line ? of unknownfile: call to core\hook\manager->add_component_callbacks()
line 348 of /lib/classes/hook/manager.php: call to array_map()
line 328 of /lib/classes/hook/manager.php: call to core\hook\manager->load_callbacks()
line 75 of /lib/classes/hook/manager.php: call to core\hook\manager->init_standard_callbacks()
line 89 of /lib/classes/navigation/views/primary.php: call to core\hook\manager::get_instance()
line 907 of /lib/pagelib.php: call to core\navigation\views\primary->initialise()
line 967 of /lib/pagelib.php: call to moodle_page->magic_get_primarynav()
line 80 of /lib/classes/navigation/output/primary.php: call to moodle_page->__get()
line 58 of /lib/classes/navigation/output/primary.php: call to core\navigation\output\primary->get_primary_nav()
line 95 of /theme/workplace/layout/drawers.php: call to core\navigation\output\primary->export_for_template()
line 1486 of /lib/outputrenderers.php: call to include()
line 1412 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line 116 of /index.php: call to core_renderer->header()

https://github.com/moodle/moodle/blob/MOODLE_403_STABLE/lib/classes/hook/manager.php#L534-L538

[kabalin]

Gosh, that is true, thanks @paulholden. I knew that it is recent in fact, but did not take into account, oh shame shame.