Should fix slsa-framework#726

Signed-off-by: laurentsimon <64505099+laurentsimon@users.noreply.github.com>
Signed-off-by: Ramon Petgrave <ramon.petgrave64@gmail.com>

# Summary

Fixes renovate config to use the
[`config:best-practices`](https://docs.renovatebot.com/presets-config/#configbest-practices)
preset rather than the `config:base` preset since `config:base` seems to
have gone away at some point.

Also fixes the `schedule` config by using the
[`schedule:monthly`](https://docs.renovatebot.com/presets-schedule/#schedulemonthly)
preset. The previous `schedule` config seems to have been invalid
because "4 am" had space between "4" and "am" (this was fixed in the
`slsa-verifier` repo on
slsa-framework/slsa-verifier#727 but was never
fixed here).

Also adds a pre-submit to run the
[`renovate-config-validator`](https://docs.renovatebot.com/config-validation/)
to ensure that renovate config is valid. This pre-submit will need to be
made required in the repository branch protection rule for `main` in the
repository settings after this PR is merged.

Fixes #3634 #404 

## Testing Process

- Run `make renovate-config-validator` to check that the config is
valid.

## Checklist

- [x] Review the contributing
[guidelines](https://github.com/slsa-framework/slsa-github-generator/blob/main/CONTRIBUTING.md)
- [x] Add a reference to related issues in the PR description.
- [x] Update documentation if applicable.
- [x] Add unit tests if applicable.
- [x] Add changes to the
[CHANGELOG](https://github.com/slsa-framework/slsa-github-generator/blob/main/CHANGELOG.md)
if applicable.

---------

Signed-off-by: Ian Lewis <ianlewis@google.com>
Signed-off-by: Ramon Petgrave <32398091+ramonpetgrave64@users.noreply.github.com>
Co-authored-by: Ramon Petgrave <32398091+ramonpetgrave64@users.noreply.github.com>