RFR: 2004: Field 'groups' in Class GitLabHost should be 'List'

[Zhao Song]

Currently, the type of field 'groups' in GitLabHost is a set. However, we would like the bot has some preference for some groups, so this field should be able to keep the order. 

The solution is to change the type this field to List.

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Commit messages:
 - SKARA-2004

Changes: https://git.openjdk.org/skara/pull/1550/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1550&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2004
  Stats: 25 lines in 4 files changed: 2 ins; 1 del; 22 mod
  Patch: https://git.openjdk.org/skara/pull/1550.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1550/head:pull/1550

PR: https://git.openjdk.org/skara/pull/1550