Rename ci_runner_machines.machine_xid to system_xid

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR renames the recently introduced ci_runner_machines.machine_xid column to system_xid. It follows the procedure outlined in https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html. The whole sequence is:

sequence	MR	documentation
1		Add the regular migration (release M)
2		Ignore the column (release M)
3	👉 Rename ci_runner_machines.machine_xid to system... (!111428 - merged)	Add a post-deployment migration (release M)

Part of #390241 (closed)

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