Ensure `getCompoundEntityRef` in `@backstage/catalog-model` normalizes its values to lowercase

[dotboris]

Hey, I just made a Pull Request!

This PR changes getCompoundEntityRef so that it returns the parts of the compound ref as lowercase. This fixes an issue I've been facing in the MyGroupsSidebarItem. This component would like to /catalog/default/Group/my-group instead of /catalog/default/group/my-group (uppercase G vs lowercase g). This ends up being a problem for us because our analytics treat those two paths as different when in reality they are not.

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