[a11y] Close button is not clickable when ConflictPicker is opened

[emoral435]

See: nextcloud/server#43667 (comment)

It seems that the close button on the ConflictPicker is not currently accessible with a mouse cursor. It does seem related to its z-index, so I will investigate this ASAP!

[susnux]

I think migrating from NcModal to NcDialog should fix this.

We had this also on photos.