Mail: Ensure that a server hostname can be set by using `network_home_url()`.

Due to the varying server setups, `$_SERVER['SERVER_NAME'];` can't reliably ensure that there will be a relevant host that could be used as the hostname in an email. Since the `network_home_url()` is set at this point, and is filterable, this should better enable emails to be sent from the server.

Fixes #25239.
Props layotte, SergeyBiryukov, nacin, sreedoap, szepe.viktor, jesin, kitchin, mensmaximus, mt8.biz, Grzegorz.Janoszka, cbutlerjr, dd32, BjornW, neodjandre, BjornW, Ipstenu, ocean90, danielbachhuber, desmith, joemcgill, jdgrimes, riasat, derekakelly, swissspidy, lilmike, tsimmons, robert.peake, DavidAnderson, cloudstek, pigdog234, BjornW, rawrly, pessoft, RedSand, seayou, dvershinin, bastho, chesio, Starbuck, afragen, ocean90, whyisjake.

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