Jenkins GitLab Branch Source plugin likely affected by branch list cache

In !61688 (merged) we introduced a short-lived cache for the branches list API to prevent excess load on Gitaly. This might have an effect on the following plugin for Jenkins, which queries that API after a push:

https://plugins.jenkins.io/gitlab-branch-source/

We should verify whether this is affected and whether we can change anything in the short-term to improve it, but the long-term solution is to finish !61692 (closed) and remove the cache when it is no longer necessary.

Some notes:

1. If there's a delay of up to 30 seconds before the Jenkins job hits the API, there's unlikely to be an issue, unless:
2. Multiple jobs are run in short succession and hit the API endpoint with identical params
3. We might be able to implement a delay somewhere
4. Alternatively we can document this quirk and its temporary nature, and instruct Jenkins users to add a 30s delay into their job run for this plugin

/cc @cptiglo who reported this on the previous MR

@sean_carroll I'm not sure who maintains the Jenkins plugin or whether we have anything to do with it, but I suspect we need to solicit feedback from people who use it first to see if this is negatively affecting them.