SOCIAL_AUTH_GOOGLE_OAUTH2_* are now not a part of 'external secret'

After the changes, introduced in https://gitlab.developers.cam.ac.uk/uis/devops/gcp-deploy-boilerplate/-/merge_requests/116, variables

    "SOCIAL_AUTH_GOOGLE_OAUTH2_KEY",
    "SOCIAL_AUTH_GOOGLE_OAUTH2_SECRET",

will no longer be available in so-called external secret.

The webapp-boilerplate template must be adjusted accordingly.