Add migration to rename ci_runner_machines.machine_xid

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR is part of a series of MRs that rename 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 of MRs is:

sequence	MR	documentation
1	👉 Add migration to rename ci_runner_machines.mach... (!111426 - closed)	Add the regular migration (release M)
2	Ignore ci_runner_machines.machine_xid column (!111427 - closed)	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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