feat(platform): Use uniform platformAutomerge option (#12069)

Existing `azureAutoComplete` and `gitLabAutomerge` options are now unified/renamed to `platformAutomerge`, and also defaulting to `true`.

BREAKING CHANGE: Azure autocomplete will now only be enabled when `automerge` is also enabled (to align with how GitLab automerge already worked). If you were using `azureAutoComplete` before without explicitly enabling Renovate `automerge` field then you will now need to do so. Platform automerge is now the default behavior whenever automerge is enabled.