docs: update v2 migration guide

[shellcromancer]

Description

This fixes examples in the v2 migration guide for sub.cnd.meta.* conditions that replace for_each conditions by removing the array parameter. Users following this guide would get Jsonnet that failed to build.

Motivation and Context

Discovered this while migrating an old configuration and following the guide resulted in a broken config.

How Has This Been Tested?

Verified that the Jsonnet could be built and passed with substation vet

Types of changes

• Bug fix (non-breaking change which fixes an issue) * of documentation*
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.