5.3.0

Move references to the yontrack/yontrack-cli repository

The repository has been transferred from nemerosa/ontrack-cli to
yontrack/yontrack-cli, which changes three things that were pinned to the
old home.

The CI credentials no longer resolve: ONTRACK_URL and ONTRACK_TOKEN came
from the nemerosa organisation and do not follow a transfer. The yontrack
organisation names them YONTRACK_URL and YONTRACK_TOKEN, as used throughout
yontrack/yontrack, so both workflows now reference those.

The release workflow pinned `--project ontrack-cli` as a literal, while
go.yml lets Yontrack derive the project from GITHUB_REPOSITORY. After the
transfer those two would resolve to different projects and the changelog
export would find no build. It now derives the project the same way, from
the repository name, so a future rename carries itself.

The README build badge and releases link pointed at nemerosa/yontrack, a
repository that has never existed - both 404ed before this change. The
GitHub setup example used the same dead path as its sample value; the main
product now lives at yontrack/yontrack.

The ontrack-cli-* release assets are deliberately left in place: one
migration at a time.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>