feat: dd-trace 5.0 to 6.0 - rename runtime id env var

[mohab-sameh]

Summary

Rename DD_TRACE_EXPERIMENTAL_RUNTIME_ID_ENABLED to
DD_RUNTIME_METRICS_RUNTIME_ID_ENABLED.

Detection Criteria

Detect the old env var in source and supported config formats.

Transformation Logic

Replace the env var name with DD_RUNTIME_METRICS_RUNTIME_ID_ENABLED.

Before / After Example

process.env.DD_TRACE_EXPERIMENTAL_RUNTIME_ID_ENABLED = "true";

process.env.DD_RUNTIME_METRICS_RUNTIME_ID_ENABLED = "true";

Notes / Edge Cases

Implemented for JS/TS source contexts. The package workflow also has a gated
AI config step for exact active occurrences in .env, YAML, Dockerfile, shell,
CI, and deployment files.