[Fix #9934] Fix configuration loading to not raise an error for an obsolete ruby version that is overridden

[dvandersluis]

Previously, when a configuration file inherits another that sets TargetRubyVersion to an obsolete version, a config validation error is raised by RuboCop. This causes an irreconcilable issue if the inherited config is shared with other projects or not editable (from upstream, etc.).

In order to fix this, the target ruby version check is moved to happen after all the config resolution steps have completed, so at that point, all the various TargetRubyVersion values have been collapsed into a single value that can be verified. This allows for any number of complicated inheritence chains without causing issues.

Fixes #9934.

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[bbatsov]

Nice fix!