Thank you for the feedback.
I tried, and I got the same message.
So, I modified the Gemfile to re-enable kerberos... and it worked!
However, the message at the end says that:
Bundle complete! 182 Gemfile dependencies, 248 gems now installed.
Gems in the groups development, test, mysql, kerberos and sqlite were not installed.
Bundled gems are installed into ./vendor/bundle.
I verified, "kerberos" wasn't in the command line. Strange. Perhaps something has automatically understood that kerbaros should be ignored, even if it is in the list of dependencies...
I'm at 8.9.11 now ! Still some effort until 8.13.3
Thanks for the support. Is the doc somewhere on a git so that I propose some modifications?