Raise the minimum supported version of Gradle to 4.4

[wilkinsona]

Since 2.0.0, we've supported Gradle 4.x (that currently spans 4.0 - 4.10 inclusive). For Boot 2.1, I think we should consider raising the minimum supported version. 4.10 may be a bit extreme but somewhere near that version should hopefully be reasonable.

@eriwen If you have a moment, any suggestions you have for a reasonable minimum version to support would be much appreciated.

[eriwen]

As of August the vast majority (80%+) of Gradle users are adopting Gradle 4.4 or higher, with 4.4 being most popular, and newer versions rising quickly. I'm afraid I don't have good data for Spring Boot users specifically at this time.

Are there particular features that you'd like to take advantage of?

[wilkinsona]

Thanks, @eriwen. The main motivator for this was to lower our maintenance burden. While it will also make it easier to adopt new features without having to worry as much about backwards compatibility, I don't have any specific new features in mind at the moment. I'm also aware that Gradle 5.0 is on the horizon and limiting our support of 4.x will hopefully put is in a better position to support 5.0 in a timely manner.

Let's go with 4.4 as a minimum for Boot 2.1.

[eriwen]

Sounds good to me. Happy to help.