Correct the fallbacks for mailer configuration (#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>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -713,7 +713,7 @@ and
 [Gitea 1.17 configuration document](https://github.com/go-gitea/gitea/blob/release/v1.17/docs/content/doc/advanced/config-cheat-sheet.en-us.md)
 
 - `ENABLED`: **false**: Enable to use a mail service.
-- `PROTOCOL`: **\<empty\>**: Mail server protocol. One of "smtp", "smtps", "smtp+startls", "smtp+unix", "sendmail", "dummy". _Before 1.18, this was inferred from a combination of `MAILER_TYPE` and `IS_TLS_ENABLED`._
+- `PROTOCOL`: **\<empty\>**: Mail server protocol. One of "smtp", "smtps", "smtp+starttls", "smtp+unix", "sendmail", "dummy". _Before 1.18, this was inferred from a combination of `MAILER_TYPE` and `IS_TLS_ENABLED`._
   - SMTP family, if your provider does not explicitly say which protocol it uses but does provide a port, you can set SMTP_PORT instead and this will be inferred.
   - **sendmail** Use the operating system's `sendmail` command instead of SMTP. This is common on Linux systems.
   - **dummy** Send email messages to the log as a testing phase.