Update CI job exit code getter/setter and apply feature flags

What does this MR do and why?

Context

Since some job metadata will be moved to new destinations (e.g. intrinsic data moved to ci_builds and immutable processing data to ci_job_definitions) we need to be able to use feature flags to read/write data to specific destinations.

This MR

Updates the exit_code getter/setter, moves them to Ci::Metadatable and applies the following feature flags:

• read_from_new_ci_destinations (Roll out: #552063 (closed))
• stop_writing_builds_metadata (Roll out: #552065)

References

• Resolves part of #552057 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #552057 (closed)