Change backoff strategy to exponential for BitbucketCloudApiClient when rate-limited

[thomas-boehm-tractive]

Change backoff strategy to exponential for BitbucketCloudApiClient when rate-limited, as a fixed wait time of 5 seconds sends too many unnecessary requests that can be avoided.

Your checklist for this pull request

• [x] Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or in Jenkins JIRA
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Did you provide a test-case? That demonstrates feature works or fixes the issue.
  • no- as I am unsure how to test this

Fixes #840

[nfalco79]

This PR is already covered by the #414 that have a long discussion. There is also consideration in issue #831 and https://issues.jenkins.io/browse/JENKINS-64418

[thomas-boehm-tractive]

This PR is already covered by the https://github.com/jenkinsci/bitbucket-branch-source-plugin/pull/414 that have a long discussion. There is also consideration in issue https://github.com/jenkinsci/bitbucket-branch-source-plugin/issues/831 and https://issues.jenkins.io/browse/JENKINS-64418

@nfalco79 does that mean I sould close this PR?

[nfalco79]

PRs should refer to an issue with the reason of changes, in this cases there is one with details. For what I see other PR has an ongoing review with the old-old maintainers but was stucks. What I mean is that this PR should takes in consideration those changes, comments and the JIRA issue to completed.