legacylibrarian: use cmd/legacylibrarian on main instead of legacy branch

[julieqiu]

We may want to update Go & Python to run cmd/legacylibrarian from main instead of cmd/librarian on the legacy-librarian branch, so we can keep our tools on a single code path ahead of migrating.

This allows all our tools use a single code path as we prepare for migration. The branches are already drifting: legacy-librarian has recent command updates, while main contains the internal/semver logic required for IBV.

Consolidating them now will simplify the migration later, so that we don't need to worry about functionality differences.

[julieqiu]

/cc @codyoss @jskeet

[jskeet]

+1 to having a single branch with legacy and non-legacy Librarian on, so long as it doesn't cause conflicts.

[codyoss]

After we tag the release we will need to update the dockerfiles to build the correct CLI. So we don't need to update a bunch of things in different place I suggest we still call the cli librarian when we build a binary there. We will also need to check to see if the cloudbuild.yamls need to be unpinned.

Additionally in the console we need to update the triggers to run on main instead of the legacy branch.