Raise the minimum supported version of Gradle to 5.6

[wilkinsona]

In Spring Boot 2.2 we retained support for Gradle 4 (specifically 4.10.x) in a deprecated form and documented that it would be dropped in a future release. I think we should do that in 2.3. At the same time we should raise the minimum support version of 5.x to something relatively modern to make it easier to support 5.x and 6.x in parallel.

[wilkinsona]

I think we should be quite aggressive here in M1 and see how it goes. I'd like to try only supporting 5.6.x and 6.x in Spring Boot 2.3.

[dreis2211]

This breaks the jdk-13-build. See #19680 .