New versions of reagent require an explicit react dependency.

Will stick to testing the oldest and newest LTS versions I
support. This greatly reduces the number of tests I have to run.

Gradle only supports the latest 2 majors. 6 is already EOL, 7 will be
relatively soon since 9 is in development.

Java 21 minimum version was 8.5. Starting to seem like 8.5 is the one
that hangs in CI a lot. Trying to drop that and go to 8.6 as a minimum
to see if it resolves the issue.

Gradle 9.0.0 enforces use of the JUnit Platform Launcher as a
dependency. This pattern works on older versions and is probably what
we'll update the docs to show.

Gradle 9 fails if a subproject doesn't exist.

Woops... We were apparently running all tests against the version of
Gradle the overall build was using. I'm sure I wasted a whole bunch of
time troubleshooting test failures before that was caused by this.

Older Gradle versions and older Java versions need older shadow plugin versions.

Forgot about dropping support for 6 and 7 as part of this.