Enables frozen for the following:

* lib/*.rb
* lib/banzai/**/*.rb
* lib/bitbucket/**/*.rb
* lib/constraints/**/*.rb
* lib/container_registry/**/*.rb
* lib/declarative_policy/**/*.rb

Partially addresses #47424.

For these kind of issues, the "reporter" field is present but zero.
In such a case, "fetch" will not return the default value, but it will
return nil.
Hence, importing fails, because the "username" field of nil is referenced

Fixes issue #44381

Signed-off-by: Rémy Coutable <remy@rymai.me>