Where a defendant seeks permission to file a successive postconviction petition alleging that his sentence was a de facto life sentence in violation of the proportionate penalties clause of the Illinois constitution, he may not rely on the decision in Miller v. Alabama, 567 U.S. 460 (2012) as sufficient cause for not raising the issue on appeal or in a prior postconviction petition, as Illinois courts had already recognized differences between minors and adults for the purpose of sentencing under the proportionate …