Update release notes for ApplicationConversionService.getSharedInstance() being unmodifiable

[dan-lind]

I'm in the process of upgrading our project from 2.4.2 to 2.5.6.
While in the progress of testing this, we started seeing a lot or errors saying "This ApplicationConversionService cannot be modified".

It looks like this is caused by us using ApplicationConversionService.getSharedInstance().addConverter() and a change that was introduced in #26088

The top bullet in the RC1 mentions this as a new feature.
https://github.com/spring-projects/spring-boot/releases?page=3

Looks like a breaking change to me? I think it would be nice if it was properly documented as such, and with at least a pointer to the documentation of how to add converters instead.

[wilkinsona]

Good catch, thank you. This should have been mentioned in the release notes.