migrate `better-auth` to `1.6`

[sacrosanctic]

https://better-auth.com/blog/1-5

[sacrosanctic]

1.5 seems a bit rough around the edges, we should wait a bit for them to squash some bugs first.

[jycouet]

Too bad in a minor.
Thx for the note

[jycouet]

Even 1.6 now https://github.com/better-auth/better-auth/releases/tag/v1.6.0