docs: remove obsolete arm64 build instructions

[MikeMcC399]

• closes Outdated instructions "Updating images to add linux/arm64" #1136

Situation

The CONTRIBUTING document section Updating images to add linux/arm64 is outdated. It lists instructions that were only possible to carry out prior to the migration of the repo to the cypress/factory process in Jan 2023.

Using the current cypress/factory process:

• locally built images use amd64 or arm64 according to the local processor environment
• images built in CircleCI are automatically built for both amd64 and arm64 for testing
• images built for publication in CircleCI are multi-architecture
• Yarn is no longer used
• There is no version-specific Dockerfile created

Change

Remove the obsolete and unusable section Updating images to add linux/arm64 from the CONTRIBUTING document.

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[jennifer-shehane]

Thanks!