Deprecate p_ci_builds_execution_configs in favor of p_ci_job_definitions
## Summary
When discussing the implementation of https://gitlab.com/gitlab-org/gitlab/-/issues/550676+, we decided to do the full migration in another Epic independently from https://gitlab.com/groups/gitlab-org/-/epics/13886+.
## Proposal
Currently, `execution_config` is passed from `lib/gitlab/ci/yaml_processor/result.rb` and deleted
in the `initialize` method of `Seed::Build` and assigned to `@execution_config_attribute`. In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/204186 (for https://gitlab.com/gitlab-org/gitlab/-/issues/550676), we did not want to make a huge change in the process; we only wanted to save the data to the `p_ci_job_definitions` table.
In this epic, we are planning to:
1. Implement reading the run steps from `p_ci_job_definitions` instead of `p_ci_builds_execution_configs`.
2. Start not saving any more data to `p_ci_builds_execution_configs`.
3. Drop the `p_ci_builds_execution_configs` table.
epic