Always flush the BIO when we send any alert

At the moment we flush the write BIO if we send a fatal alert, but not a
warning one. This can mean the warning is never sent if we never do another
write and subsequently flush the BIO. Instead we should just always flush
after writing an alert.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3432)