SpringBoot Allow JSON keys containing non-canonical characters from CF environment to be handled as a single path segment

We need to have another go at #18915 without causing the problems described in #20343 again.

Comment From: wilkinsona

This is labelled as an enhancement but assigned to 2.3.x. I wonder if it belongs in 2.4.x or if we should consider it to be a bug as the original attempt was.