SpringBoot Remove default spring.mongodb.embedded.version

Removes the previous default setting for spring.mongodb.embedded.version of 3.5.5.

Fixes gh-26220

Comment From: wilkinsona

The upgrade instructions in the releases notes should mention that spring.mongodb.embedded.version now needs to be set and explain why that's the case.

Comment From: wilkinsona

Thanks very much, @bono007.