MINOR: Add 4.0 branch to CI

[dajac]

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[mumrah]

I think we'll need to extend the cancel-in-progress logic to check for refs/heads/4.0 as well.

We can probably leave the two "gradle-cache-" properties as they are. This will effectively disable the cache on release branches, but since we are already at our cache usage limit so this is probably desired.

Edit: actually we'll want gradle-cache-read-only to be true for release branches, so that will need to be updated

[mumrah]

For the two Gradle cache properties, we'll get

gradle-cache-read-only: true
gradle-cache-write-only: false

for 4.0. This means we'll load the closest matching cache (the branch point, I guess?) which will basically mean starting from scratch (which is fine). Since we have many fewer builds and PRs for release branches, we can probably disable caching altogether for release branches. That doesn't need to happen in this PR though.

LGTM

[mimaison]

We also need to update the release process document to include this step, and remove the jenkins configuration section.