Update URLs to latest versions

[sebrandon1]

Update URLs to their latest version counterparts.

Documentation reference updates:

• Updated the "What can you change with machine configs?" documentation link in machine-config-irreconcilable-changes.md from 4.18 to 4.19 to reflect the latest supported fields.
• Changed the customized stub Ignition config documentation link in manage-boot-images.md from 4.18 to 4.19.
• Updated the boot image update timeline table in manage-boot-images.md to reference 4.19 documentation for GCP and AWS platforms, ensuring all milestone links are consistent with the latest release.
• Updated the bare metal ISO installation documentation link in manage-boot-images.md from 4.18 to 4.19 for accuracy in describing environments not using the MCS.

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PR needs rebase.

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