FFM-9804 Fix percentage rollout prefix

[erdirowlands]

What

Updates percentage rollout prefix to align with other SDKs.

Why

The prefix is backwards in this SDK, and Ruby, compared to our other SDKs.

Testing

Manual - compared hash for given target with Golang SDK

[andybharness]

looks good, note we'll need to increment the main version number and add a note to the release notes, just so any existing customers know that buckets may shift

[erdirowlands]

looks good, note we'll need to increment the main version number and add a note to the release notes, just so any existing customers know what buckets may shift

Cheers @andybharness will do that now, note I just changed the base to main-v2 there in case we need to get bug fixes in to main in the meantime. Once we are ready to release, we can merge main-v2 back