fix(licensing, v1.2): restore the ScalaPB notice on the vendored scalapb.proto - #8868

…oto (#8806)

### What changes were proposed in this PR?

Per the note at the end of #8802,
`common/workflow-core/src/main/protobuf/scalapb/scalapb.proto` is
vendored
from ScalaPB (Apache-2.0) but carried the standard ASF header — the same
class of problem as #8775. Mirroring that fix:

- `scalapb.proto`: ASF header → a vendored-from notice (upstream ships
the
file with no header; its license is the repository-level Apache-2.0, so
no
  separate license copy is needed);
- root `LICENSE`: a ScalaPB entry next to the other Apache-2.0
third-party
  entries ("this LICENSE file");
- `.licenserc.yaml`: exempt the file from the ASF-header check.

Comment-only change to the proto; protoc output is unaffected. This
addresses
only the vendored-file item in #8802 — the build-time-generation
proposal
stays open there.

### Any related issues, documentation, discussions?

Addresses the vendored scalapb.proto item in #8802. Same pattern as
#8775/#8776.

### How was this PR tested?

License-metadata-only change; the Check License Headers CI job verifies
the
exemption. The file is identical on main and both release branches.

### Was this PR authored or co-authored using generative AI tooling?

(backported from commit 1df2c56)

Generated-by: Claude Code (claude-fable-5)