Update the Loader Script to support v8

[rodolfoBee]

Today, the Loader Script defaults to & supports v7 of the SDK.

We want to also support v8 there. We should implement this in a few steps:

### Tasks
- [x] Allow to select v8 option, guarded behind a feature flag - initially, continue to serve v7 when this option is selected
- [x] Update Loader Script to emit v8-compatible code (?) when v8 is selected.
- [x] Extend feature flag to show this option to all users, but keep v7 as default
- [ ] When verified that it works, make v8 the new default

[Lms24]

This is on our radar but we held off from it with the initial 8.0.0 launch to reduce the number of moving parts. We're aware and will tackle this soon (TM)

[lforst]

All of this is prepared, we just need to flip the switch when we feel like we are ready.

[Lms24]

The loader script is blocked on #12188