Remove legacy contributing process docs

What does this MR do and why?

The PROCESS.md file is outdated and duplicates information available elsewhere in the docs.

This MR removes it and attempts to remove any references too.

Related to Move PROCESS.md under the doc directory (#23506 - closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.
• All open threads have been resolved