Drop "@" from email sender to avoid spam filters (#29109)

Drop "@" from email sender to avoid spam filters (#29109)

Commit 360b3fd17c (Include username in email headers (#28981),
2024-02-03) adds usernames to the From field of notification emails in
the form of `Display Name (@username)`, to prevent spoofing. However,
some email filtering software flags "@" in the display name part of the
From field as potential spoofing, as you could set the display name part
to another email address than the one you are sending from (e.g.
`From: "apparent@email-address" <actual@email-address>`). To avoid
being flagged, instead send emails from `Display Name (username)`.

Closes: #29107
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Co-authored-by: Giteabot <teabot@gitea.io>