Fixes #38040 - Clear retain_version_count when mirroring policy is not additive

[sjha4]

What are the changes introduced in this pull request?

1. On the API, clear out retain_package_versions_count when mirroring policy is anything other than additive
2. Add a validation on root model to make sure retain_package_versions_count is only set when mirroring policy is additive.

Considerations taken when implementing this change?

What are the testing steps for this pull request?

1. Create a yum repo with some retain_package_versions_count and mirroring policy as Additive
2. Sync
3. Now update mirroring policy to Content only/Complete and sync again.
4. You'll see an error.
5. In console if you check repo.root.retain_package_versions_count it will still have the value from step 1

With this change,
In step 4 you'll not see the sync error
In console if you check repo.root.retain_package_versions_count you'll see nil

We can also test the validation by doing so in console:

repo.root.mirroring_policy = "mirror_complete"
repo.root.retain_package_versions_count = 3
repo.root.valid? # false
repo.root.errors.messages # {:retain_package_versions_count=>["cannot be set for repos without 'Additive' mirroring policy."]}