Remove code deprecated in Spring Boot 2.3

[snicoll]

[spencergibb]

We're hoping that Spring Cloud 2020.0 can be compatible with boot 2.4 and 2.5. I'm not aware of any usage of deprecated apis, just want to give a heads up.

[snicoll]

As the title suggests, we will only remove code that was deprecated in 2.3. We are doing this as early as possible to adapt if that's causing an issue.

[spencergibb]

Indeed. Like I said, haven't seen anything yet, but will see if we have any.