Update stable/11 from 11.2-STABLE to 11.3-PRERELEASE, marking the
official start of the code slush for the 11.3-RELEASE cycle.

Set the default mdoc(7) version to 11.3, and update the clang(1)
TARGET_TRIPLE to reflect 11.3.

Update the default pkg(8) repository to the 'quarterly' branch to
prevent further 11.3 builds from downgrading packages when invoking
'pkg upgrade' for the duration of the cycle. This will be reverted
on the stable/11 branch once releng/11.3 is branched. [*]

Approved by: re (implicit)
MFC after: 6 weeks
MFC note: reminder to revert pkg(8) default repository [8]
Sponsored by: The FreeBSD Foundation