Fix #21370

This PR incorporates 2 fixes:

1. VALARMs shouldn't be propagated to attendees via iTIP (not mandatory but recommended by RFC).
2. Multiple email notifications are dispatched because there is no differentiation between when the owning user is the organizer of an event and when the owning user is an attendee.
3. The email of the organizer was never extracted properly because the usage of strcasecmp($organizer->getValue(), 'mailto:') !== 0 was wrong. This lead to the Reply-To header not being set. -> Moved to a separate PR because it's not related.

How to test

Preparations

1. Have at least 2 users.
2. Configure your instance to be able to send emails.
3. Enable instant event reminder sending: occ config:app:set dav sendEventRemindersMode --value occ
4. Apply the following patch to make testing easier (by ignoring actual trigger date and time):

diff --git a/apps/dav/lib/CalDAV/Reminder/Backend.php b/apps/dav/lib/CalDAV/Reminder/Backend.php
index b0476e9594c..74a980d3f91 100644
--- a/apps/dav/lib/CalDAV/Reminder/Backend.php
+++ b/apps/dav/lib/CalDAV/Reminder/Backend.php
@@ -66,7 +66,6 @@ class Backend {
 		$query = $this->db->getQueryBuilder();
 		$query->select(['cr.*', 'co.calendardata', 'c.displayname', 'c.principaluri'])
 			->from('calendar_reminders', 'cr')
-			->where($query->expr()->lte('cr.notification_date', $query->createNamedParameter($this->timeFactory->getTime())))
 			->join('cr', 'calendarobjects', 'co', $query->expr()->eq('cr.object_id', 'co.id'))
 			->join('cr', 'calendars', 'c', $query->expr()->eq('cr.calendar_id', 'c.id'));
 		$stmt = $query->execute();

Routine

1. Create a new event with at least one email reminder (trigger date and time don't matter).
2. Add the other user as an attendee.
3. Save the event.
4. Trigger reminder sending: occ dav:send-event-reminders
5. Observe that each users receives a single reminder email -> Duplicate emails fixed ✔️
6. Login with the other user (which is an attendee).
7. Observe that there are no reminders on the incoming event -> ALARMS shouldn't be propagated (recommended by the RFC) ✔️
8. Add a "private" email reminder to my local copy.
9. Save the event and trigger reminder sending again.
10. Observe that only I (the attendee) receive an email -> Only the email reminders of organizers should be sent to all attendees ✔️