RFR: 1728: Should iterate over all issues in PullRequestWorkItem in CSR bot

[Zhao Song]

While testing [SKARA-1714](https://bugs.openjdk.org/browse/SKARA-1714) on the staging environment, I discovered a bug in the PullRequestWorkItem in the CSR bot. The issue is that the for loop does not always iterate over all CSR issues for a pull request. 

For example, if we have two active CSR issues (csr1 and csr2) for one pull request, and we withdraw csr2, the loop will exit early after iterating over csr1 and CSR_UPDATE_MARKER will not be added, which means the PR body will not be updated and the status change of csr2 will not be displayed. 

Also, if we find a withdrawn pr, we need to add CSR_UPDATE_MARKER to the pr body.

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Commit messages:
 - SKARA-1728

Changes: https://git.openjdk.org/skara/pull/1449/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1449&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1728
  Stats: 34 lines in 2 files changed: 6 ins; 8 del; 20 mod
  Patch: https://git.openjdk.org/skara/pull/1449.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1449/head:pull/1449

PR: https://git.openjdk.org/skara/pull/1449