RFR: 1817: 'WIP merge requests' renamed to 'draft merge requests' in GitLab

[Zhao Song]

Since 'WIP' is deprecated in GitLab, we need to update methods related with 'WIP'.

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Commit messages:
 - SKARA-1817

Changes: https://git.openjdk.org/skara/pull/1472/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1472&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1817
  Stats: 28 lines in 4 files changed: 22 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/skara/pull/1472.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1472/head:pull/1472

PR: https://git.openjdk.org/skara/pull/1472