Previously the following syntax would fail in the linter with an
error 500:

```
include:
  template:
    - License-Management.gitlab-ci.yml
    - Dependency-Scanning.gitlab-ci.yml
    - SAST.gitlab-ci.yml
```

Now the error will call out specifically that the value
is not a string.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66605