Copy webapp and GCP deploy boilerplate renovate configs into this repo

See webapp-boilerplate#150 (closed) and https://gitlab.developers.cam.ac.uk/uis/devops/infra/gitlab-project-factory/-/merge_requests/652#note_1811358.

Create common/webappBoilerplate.json and common/gcpDeployBoilerplate.json in this repo which copies https://gitlab.developers.cam.ac.uk/uis/devops/webapp-boilerplate/-/blob/master/renovate.json?ref_type=heads ans https://gitlab.developers.cam.ac.uk/uis/devops/gcp-deploy-boilerplate/-/blob/master/renovate.json?ref_type=heads respectively.

The idea is that one can then replace the renovate config in all the boilerplate repos with a single one like:

{
    "$schema": "https://docs.renovatebot.com/renovate-schema.json",
    "extends": [
       "local>renovate-config//common/webappBoilerplate"
    ]
}

This helps keep the boilerplate templates drier by reducing the amount of config copy-pasted between them.