Support Gradle 6.3+

[clemstoquart]

The upcoming Spring Boot 2.3 release will only support Gradle 6.3 or later. IMO we should do the same.

What do you think ?

[celcius112]

They say they will also support version 5.6, but might drop support for 6.0, 6.1 and 6.2.

Also I don't think this plugin should be coupled with Spring Boot's version. Continuing to support 5.6+ seems reasonable to me since we do not seem do use any APIs present only in 6.x.

[saugereau]

I think that we should wait the final decision, and if they also support version 5.6, but might drop support for 6.0, 6.1 and 6.2, we should do the same for this plugin.

[clemstoquart]

The primary goal is to reduce our maintenance cost. We won't test on Gradle 5.6 but it doesn't mean that it won't work.

[clemstoquart]

Spring boot will raise the minimum supported version of Gradle to 6.x in SB 2.5 spring-projects/spring-boot#24225