`packager.py` parsed the RHEL major version out of a `build_os` name with a
single-digit regex, which assumed the legacy major+minor form (`rhel93`). On
RHEL 10+ the names are major-only (`rhel10`), so the parse yielded `1`: rpms
were named `.el1` instead of `.el10`, and the yum repo path was `redhat/1`
instead of `redhat/10`. That breaks version selection for users of our repos.

Changes:

• Added `rhel_major_version(build_os)` in `buildscripts/packager.py`, which
  handles both the legacy single-digit form (`rhel93` -> `9`) and the
  major-only form (`rhel10` -> `10`). It raises on an unrecognized name rather
  than silently truncating, so a future naming change fails loudly in the
  packaging task instead of shipping mislabeled rpms.
• `Distro.repo_os_version` and `Distro.release_dist` now use the helper.
  `release_dist` was restructured into a single if/elif chain whose final
  `else` is the non-RHEL passthrough, and its docstring updated to match.
• Existing RHEL build_os names (`rhel55` through `rhel93`) map to the same
  values as before, so this is backward compatible for every currently
  published platform; only RHEL 10+ output changes.

Testing: added cases to `buildscripts/tests/test_packager.py` covering
`release_dist` (`rhel55`/`rhel80`/`rhel93`/`rhel10`), `repo_os_version`
(`rhel79`/`rhel93`/`rhel10`), and rejection of a malformed name.
`bazel run //buildscripts/tests:test_packager` passes (10 tests).