Principal search by display name case insensitive

[PVince81]

Adjust principal search to be case insensitive even with exact match.
This aligns the behavior to how the search also works in
Collaborators/UserPlugin

This aligns the behavior with https://github.com/nextcloud/server/blob/bugfix/noid/principal-search-case-insensitive-dn/lib/private/Collaboration/Collaborators/UserPlugin.php#L180

To test:

1. Disable share autocomplete to trigger exact match mode
2. Create an event
3. Type in the exact display name of a user, but with different casing

Before the fix: no results
After the fix: one result found

[miaulalala]

Nice, this also brings us more into line with RFC3774;

The actual search method (exact matching vs. substring matching vs,
prefix-matching, case-sensitivity) deliberately is left to the server
implementation to allow implementation on a wide set of possible user
management systems. In cases where the implementation of
DAV:principal-property-search is not constrained by the semantics of
an underlying user management repository, preferred default semantics
are caseless substring matches.

[PVince81]

/backport to stable23

[PVince81]

/backport to stable22