Unclear description about old branches discard on the multi-branch Pipeline setting page

Created by: LinuxSuRen

Describe the bug

Screenshot from Jenkins:

Screenshot from DevOps page:

This feature will remove those plugins that have been deleted from the git repository.

Typo

Discard old branch should be Discard old branches

Default values improvement

I suggest that we change the following default values:

• Days to keep old branches be 7
  • keep the old branches for a week
• Max number of branches to keep be 5

/area devops /priority high /help /milestone v3.2

Versions used(KubeSphere/Kubernetes) KubeSphere: latest

Environment This is not an environmental issue.

To Reproduce Steps to reproduce the behavior:

1. Go to the DevOps page
2. Create a multi-branch Pipeline
3. Check the description

Expected behavior Having a clear and correct description.