OSDOCS-15960_2_RN#replacing TP with GA

[brendan-daly-red-hat]

Version: 4.21

Issue: https://issues.redhat.com/browse/OSDOCS-15960

Link to docs preview:

• Installing a cluster on Google Cloud Platform (GCP) with a user-provisioned DNS is generally available
• Installation Technology Preview features

QE review:

• [ ] QE has approved this change.

Additional information: