CNV#72929: RN update for GC GA

[ousleyp]

Version(s): 4.21 RNs

Issue: CNV-72929

Link to docs preview:

QE review:

• QE has approved this change.

Additional information:

• Adding a basic RN for GC support; update for cloud platforms support table is CNV#72929: cloud platform table update for GC GA #107835. Can also add a link to a knowledge base article, but the current one is outdated and the one I created from Noam's doc is unpublished, so I left it out for now. The link is in the cloud platforms table though.
• [Docs] Also adding a file for 4.21.1 z-stream release updates (hoping to establish this as the practice going forward in order to avoid using illegal headings. This is based on the OCP core docs).

[openshift-ci]

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