CI proposal: switch to renovate

At the moment this repo uses `dependabot` for semi-automated dependency bumps for both Go and JS dependencies. For JS dependencies this is not working anymore, because `dependabot` only bumps package versions in the respective `package.json` file but not in the pnpm lock file. Since we're installing dependencies in CI with frozen lockfile (i.e. pnpm lockfiles can't be updated in CI, only the current state of the lock file is allowed) the JS depency bumps by `dependabot` will always fail with an out of date lock file error. One example for this is https://github.com/owncloud/ocis/pull/4990 where I needed to update the pnpm lock file myself to get green CI (well, if it weren't for flaky tests).

I'd like to propose to switch to `renovate` entirely, i.e. for both Go and JS dependencies. The `web` repo already uses `renovate` (and our sysadmin team for a long time anyway). As an alternative we could run `dependabot` for Go and `renovate` for JS dependencies in parallel, but I'd like to avoid that complexity. So the first question is if we can switch to `renovate` entirely.

cc @micbar @wkloucek @C0rby