Bumping default version of OpenNext to 3.9.14

[leanderiversen]

Bumping the default version of OpenNext to 3.9.14, after an issue reported was here: #6334 (comment)

[vimtor]

@leanderiversen the new version fails for next.js 14:

Next.js version : 14.2.15
OpenNext v3.9.14
ERROR Next.js version 14.2.15 is not supported by the Next.js team.
Major versions are supported for 2 years from their release date.
Unsupported releases may contain unpatched security vulnerabilities.

unsure about what's the best way to proceed here, we want to provide good defaults but i don't want to break existing deployments on a thing like this

so either we downgrade to the previous version before #6334 or add some message telling the user they need to set the openNextVersion to "3.6.6"

ideally we would detect this and choose in order:

1. the specified version
2. if next.js version 14 → 3.6.6
3. if next.js version 15 → 3.9.14