Backport #21945
Unfortunately the fallback configuration code for [mailer] that were
added in #18982 are incorrect. When you read a value from an ini section
that key is added. This leads to a failure of the fallback mechanism.
Further there is also a spelling mistake in the startTLS configuration.
This PR restructures the mailer code to first map the deprecated
settings on to the new ones - and then use ini.MapTo to map those on to
the struct with additional validation as necessary.
Ref #21744
Signed-off-by: Andrew Thornton <art27@cantab.net>
888384a631