The code was written at the time only one lockfile version was being
explicitly tagged (1.1.0), and all the other older ones were untagged.
In introducing the 1.2.0 format for external hashes, the current version
check still took place, but no provisions were made to explicitly check
between older or newer versions. As such, rebar3 would warn (prompting
people to upgrade) every time a lockfile on the 1.1.0 version (which is
likely over 99% of those in the wild as of today) would be encountered,
we'd prompt people to upgrade.

This patch introduces a constant for the list of supported tagged
versions for config/lock files, which allows to drop warnings when
rebar3 can transparently upgrade the lock file for the user.

It pre-supposes a potential "unsupported lock file" error could be added
in the future if a config format is not newer but in fact explicitly
deprecated. This isn't implemented but the current logic works towards
making that support possible.