[sdk-platform-java] Protobuf Compatibility CI can populate defaults from Libraries-Bom

The default values for the the Protobuf source and binary compatibility values are set [here](https://github.com/googleapis/sdk-platform-java/blob/769dd631d65ef920a46976b42404a767e14db15f/.github/workflows/downstream_protobuf_compatibility_check_nightly.yaml#L44). For now, these are hard coded since upgrading the Protobuf-Java runtime is done manually.

See if these values can be determined automatically from reading the Protobuf version from the Libraries-Bom and Libraries-Bom-protobuf3 files and used a defaults

从第 44 行的 .github/workflows/downstream_protobuf_compatibility_check_nightly.yaml 开始，然后定位提供 Protobuf 版本的 Libraries-Bom 和 Libraries-Bom-protobuf3 文件。跟踪兼容性检查如何使用它们的默认值。完成的标准是源兼容性值和二进制兼容性值能够自动推导，并且 nightly workflow 仍能正确使用它们。

由索引模型根据 Issue 内容生成。