Oneplus 6(T): Update copy-firmware instructions

[samueldr]

Two scenarios:

• Installing LineageOS intrinsically handles copying the firmware
• We continue recommending the manual approach

The template that was used previously is here:

• https://github.com/LineageOS/lineage_wiki/blob/b946596997ee9eaf5f40a551dae2e4019112c016/_includes/templates/device_specific/before_lineage_install_ab_copy_partitions.md

We probably want to check out and see if LineageOS 21 indeed handles that. In this scenario we might prefer recommending the user does a complete LineageOS install following the wiki page, and then ask the user to verify their phone works as expected features-wise on LineageOS 21.

Referring to the old template would only be done if just installing LineageOS 21 is not sufficient.