Upgrade to Gradle 5.3.x

[sbrannen]

Gradle 5.3 has been released: https://docs.gradle.org/5.3/release-notes.html

We should consider upgrading the Spring Framework build to Gradle 5.3.x during Spring Framework 5.2 development.

[snicoll]

Looks sensible to me

[sbrannen]

In light of the fact that Gradle 5.3 has been released, I think we should skip 5.2 and go straight to 5.3.

Any objections?

[sbrannen]

As a side note, Gradle 5.3 finally has built-in support for Maven optional dependencies via feature variants.

[snicoll]

Any objections?

I can't possibly see how anyone would object considering this targets the first milestone of the next feature release.