Update ConfigObsoletion so that parameters can be deprecated but still accepted

[dvandersluis]

In #9098 it was desired to change some parameters and have a deprecation warning be shown but still accept the old parameters. ConfigObsoletion previously would just error if anything was obsolete; this adds a new severity: :warning option to the OBSOLETE_PARAMETERS hash, for which issues will be warned about but rubocop will still complete.

I don't have any more tests for this because there is nothing to add to the hash until #9098 is merged, so I'll add them in there once this is accepted.

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[dvandersluis]

/cc @bbatsov

[bbatsov]

Looks good! Thanks!