RFR: Gradle cleanup [v2]

[Clayton Walker]

> I've been doing some development, and came across a few warnings during build.
> 
> These are, [Project::task](https://docs.gradle.org/current/javadoc/org/gradle/api/Project.html#task(java.lang.String)) is now deprecated, avoiding Project::mkdir, project::delete and Project::getProjectDir as they cannot work under configuration-cache.

Clayton Walker has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Fix running with configuration-cache enabled

-------------

Changes:
  - all: https://git.openjdk.org/jextract/pull/283/files
  - new: https://git.openjdk.org/jextract/pull/283/files/d576694e..c79d28e5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=283&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=283&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/283.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/283/head:pull/283

PR: https://git.openjdk.org/jextract/pull/283