Remove use of our custom patched Symfony mailer

[ssddanbrown]

Description

Should now be able to use Symfony 7.3 mailer now that my changes have been included.

[BIGWALDOR]

Hey @ssddanbrown, is it possible to get a bit more context on this?

[ssddanbrown]

Hi @BIGWALDOR,
The context is that I was maintaining some custom changes to enforce some level of TLS usage, for backwards compatibility with older versions of BookStack which used a different mailer lib.
I since provided these changes upstream: symfony/symfony#59479
Therefore we don't need to use a customized branch and can instead switch to the original version of the mailer library.