Use standardized maintainer metadata property value

The `maintainer` property of the library.properties metadata file defines the entity responsible for maintenance of the
library.

In the libraries that are under Arduino's maintainership, it is standard practice to set the `maintainer` property to
the general purpose value "Arduino <[email protected]>".

Previously, instead of that standard value, this library's `maintainer` property defined a specific developer as the
maintainer. This is prone to "bit rot", as the scope of each individual's work may change over time. It is unlikely that
an individual developer team member will remember to update the metadata values at such time as they are no longer able
to dedicate themselves to maintaining the library. So the metadata is changed to use the standard value.

Author: per1234

library.properties

@@ -1,7 +1,7 @@
 name=NTPClient
 version=3.2.1
 author=Fabrice Weinberg
-maintainer=Fabrice Weinberg <[email protected]>
+maintainer=Arduino <[email protected]>
 sentence=An NTPClient to connect to a time server
 paragraph=Get time from a NTP server and keep it in sync.
 category=Timing