[Bug]: CalDav iMIP does not distinguish between a canceled event and a withdrawn invitation

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

May be related to #29620.

1. When an organizer deletes an event in the nextcloud calendar app, the DAV server sends an iMIP CANCEL message that does not contain STATUS:CANCELLED as required by RFC 5546 3.2.5:

2. When an organizer removes an attendee from an event in the nextcloud calendar app, the DAV server correctly sends a CANCEL message without including STATUS. However, the content of the email message incorrectly says,

Subject: Cancelled: {eventtitle}

{eventtitle} has been canceled.

when it should say something like (since the event is still occurring, but the addressee has been disinvited):

Subject: Invitation Withdrawn: {eventtitle}

This invitation to {eventtitle} has been withdrawn.

(That seems a little less abrubt than the equally accurate "You've been disinvited.")

Also, the spelling of "canceled" is inconsistent - in EN-US it should be "canceled", and in EN-UK it should be "cancelled" (even the RFC is inconsistent on this).

Expected behavior

1. The iMIP message sent when the organizer deletes an event should contain both METHOD:CANCEL and STATUS:CANCELLED.

2. The iMIP message sent when the organizer removes an attendee from an event should not state that the event is canceled, it should state that the invitation is withdrawn (or similar).

3. Canceled should be spelled consistently in the iMIP email body and subject line (propose: canceled for EN-US and cancelled for EN-GB)

Nextcloud Server version

28

Trace the DAV server's iMIP CANCEL generation and the email subject/body handling, using RFC 5546 section 3.2.5 as the protocol reference. Done means event deletion includes STATUS:CANCELLED, attendee removal describes a withdrawn invitation, and cancellation spelling is consistent for EN-US and EN-GB.