nixos/doc/rl-2511: document renovate major upgrade

- `gitversion` was updated to 6.3.0, which includes a number of breaking changes, old configurations may need updating or they will cause the tool to fail to run.

  See the [6.0.0 release notes for GitVersion](https://github.com/GitTools/GitVersion/releases/tag/6.0.0) for details on the breaking changes, [the documentation on the configuration format](https://gitversion.net/docs/reference/configuration) for the new configuration specification, and [the documentation on version variables](https://gitversion.net/docs/reference/variables) for what is now supported.

- `renovate` was updated to v40. See the [upstream release notes](https://github.com/renovatebot/renovate/releases/tag/40.0.0) for breaking changes.

- `renovate` was updated to v41. See the upstream release notes for [v40](https://github.com/renovatebot/renovate/releases/tag/40.0.0) and [v41](https://github.com/renovatebot/renovate/releases/tag/41.0.0) for breaking changes.

- The `boot.readOnlyNixStore` has been removed. Control over bind mount options on `/nix/store` is now offered by the `boot.nixStoreMountOpts` option.