Raspberry Pi 32-bit deprecation: Update GitLab's documentation

Update GitLab's documentation to reflect that Raspberry Pi 32-bit is being deprecated.

Acceptance Criteria

• Update the document Running on a Raspberry Pi to indicate that we only support 64-bit OS and provide instructions on how to upgrade from 32-bit to 64-bit based on these notes
• Update #5673 (closed)
• Update the install page to reflect these changes
  • work is being done in gitlab-com/marketing/digital-experience/buyer-experience#4461 (closed), marking complete for this issue