xen-blk{back,front}: Update contact points for buffer_squeeze_duration_ms and feature...

xen-blk{back,front}: Update contact points for buffer_squeeze_duration_ms and feature_persistent

SeongJae is currently listed as a contact point for some blk{back,front}
features, but he will not work for XEN for a while.  This commit
therefore updates the contact point to his colleague, Maximilian, who is
understanding the context and actively working with the features now.

Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20220420072734.1692-1-sj@kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>