Upstream flatbuffers module@25.12.19.envoy to BCR

[phlax]

Module

• flatbuffers
• Version: 25.12.19.envoy
• BCR: ✅ exists (25.9.23 - older)
• Source: https://github.com/google/flatbuffers

Checklist

• PR 25.12.19 to BCR
• Update envoy to use BCR version when landed

Notes

• Toolshed has December 2025 version, BCR has September 2025
• flatbuffers has good upstream Bazel support