DOC-2418: Copy edits to `migration-from-6x.adoc`, and replaced version (num) with variable.

[kemister85]

Ticket: DOC-2418

Site: Staging branch

Changes:

• Copy edits to migration-from-6x.adoc
• Replaced version (num) with variable.

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/

Review:

• [ ] Documentation Team Lead has reviewed