Bug#981542: I could not reproduce this error in a clean install

[Pirate Praveen]

On Tue, 02 Feb 2021 14:26:24 +0530 Pirate Praveen 
<prav...@onenetbeyond.org> wrote:
> Reproduce the issue during upgrade on a clean machine. Installed 
gitlab
> 13.5.7 then tried to upgrade to 13.6.5 and the error happened.
>
> If anyone would like to have a look I can share access to a test
> machine.
Found a work around which involves adding back the missing method to 
activemodel but fixing it the right way would be better.

See https://gitlab.com/gitlab-org/gitlab/-/issues/300882#note_500826284