Remove legacy columns from ci_builds

Overview

There are bunch of deprecated columns in ci_builds, we should identify and remove them as part of &4685 (closed).

Columns

Column	Type	Collation	Nullable	Default	Deprecated?
id	integer		not null	nextval('ci_builds_id_seq'::regclass)	❔
status	character varying				❔
finished_at	timestamp without time zone				❔
trace	text				❔
created_at	timestamp without time zone				❔
updated_at	timestamp without time zone				❔
started_at	timestamp without time zone				❔
runner_id	integer				❔
coverage	double precision				❔
commit_id	integer				❔
commands	text				❔
name	character varying				❔
options	text				❔
allow_failure	boolean		not null	false	❔
stage	character varying				❔
trigger_request_id	integer				❔
stage_idx	integer				❔
tag	boolean				❔
ref	character varying				❔
user_id	integer				❔
type	character varying				❔
target_url	character varying				❔
description	character varying				❔
artifacts_file	text				❔
project_id	integer				❔
artifacts_metadata	text				❔
erased_by_id	integer				❔
erased_at	timestamp without time zone				❔
artifacts_expire_at	timestamp without time zone				❔
environment	character varying				❔
artifacts_size	bigint				❔
when	character varying				❔
yaml_variables	text				❔
queued_at	timestamp without time zone				❔
token	character varying				❔
lock_version	integer			0	❔
coverage_regex	character varying				❔
auto_canceled_by_id	integer				❔
retried	boolean				❔
stage_id	integer				❔
artifacts_file_store	integer				❔
artifacts_metadata_store	integer				❔
protected	boolean				❔
failure_reason	integer				❔
scheduled_at	timestamp with time zone				❔
token_encrypted	character varying				❔
upstream_pipeline_id	integer				❔
resource_group_id	bigint				❔
waiting_for_resource_at	timestamp with time zone				❔
processed	boolean				❔
scheduling_type	smallint				❔

Notes

The stage column is known to take a lot of space, and it might not be needed anymore after we enable ci_atomic_processing FF.