Update v2 Migration Guide to move away from use of @apply

[endigo9740]

Link to the Page

https://www.skeleton.dev/docs/get-started/migrate-from-v2

Describe the Issue (screenshots encouraged!)

It's been brought to my attention the Remove @apply section from the pre-release migration guide never made it to the v2 migration guide. We should aim to update and append this asap.

https://github.com/skeletonlabs/skeleton/discussions/3242

[okeken]

👋 Hi! I'd like to work on this documentation issue.

@endigo9740 can you assign me this task? Cheers!

[endigo9740]

@okeken done, thanks