Support multiple license keys in development or test

https://gitlab.com/gitlab-org/gitlab-development-kit/-/merge_requests/3170
was merged recently to set `GITLAB_LICENSE_MODE` to `test` by default
in the GitLab Development. GitLab IT has been moving towards
generating licenses from the staging customer portal for development
(https://gitlab.com/gitlab-org/gitlab-development-kit/-/issues/1824). However,
developers who had licenses generated with the production portal had
to configure `GITLAB_LICENSE_MODE` correctly with `prod` or EE
features would quietly stop working.

To avoid this, we take advantage of a new feature in gitlab-license to
support multiple decryption keys. We always load the production key as
the main key, but if we're in a non-production environment or if
`GITLAB_LICENSE_MODE` is set to `test`, use the test decryption key as
a fallback.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/414723

Changelog: added