Add minimum GitHub token permissions for workflows

[joycebrum]

@akarnokd can I bring up this topic (#7469) again and perhaps suggest a PR with the permission that were missing in this one? It is really a relevant topic on supply-chain security that prevents any threats and attacks.

But before I would like to understand what didn't work on https://github.com/ReactiveX/RxJava/actions/runs/2343714818/jobs/3517587271 because for me it shows as success.

Looking into gradle_snapshot.yml and gradle_release.yml I've noticed that perhaps the permission that were missing was a contents: write to run this command

RxJava/push_javadoc.sh

Line 118 in ed674be

git push --quiet --set-upstream origin-pages gh-pages

Let me know if a PR is welcome and I'll try testing it before submit it, although I think it will be difficult considering sonatype access used.

[akarnokd]

The build doesn't fail if it can't push the javadoc back.

Alright, let's try your suggestion via PR.