default-versions: add rust-bootstrap to RUST section

lang/rust-bootstrap is another potential option for RUST.
document the value in the comment so others might consider its use.

lang/rust-bootstrap has more tunable knobs which allow for more minimal
builds of the rust toolchain than lang/rust -- which may be desirable
for appliance or iterative builds.

Approved by: portmgr (bapt)
Reviewed by: allanjude, bapt
Differential Revision: https://reviews.freebsd.org/D42052