RFR: 1450: Jcheck status message can be too big for Gitlab

[Zhao Song]

SKARA-1450 describes the problem that the comment adding to a GitLab merge request can be too big to add and leads to fail. 

To fix it, I add the function `limitBodySize()` to `addComment()` and `updateComment()` in `GitLabMergeRequest`, the function will check the size of the comment and  the comment will be cropped if it is too big.

I also found that GitHub pull request might have the same problem, so I did the same change to `GitHubPullRequest`.

Currently, I set the comment max size to 20000, and I don't know whether it is fine. I think it could not be too small because the user wants to see more information and it should not be big because if it is too big, the page loading process will be slow when the user tries to open the pr.

[Maximum length for the comment body in GitHub PR](https://github.com/orgs/community/discussions/27190)
[My test in Playground](https://github.com/openjdk/playground/pull/99)

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Commit messages:
 - SKARA-1450

Changes: https://git.openjdk.org/skara/pull/1403/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1403&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1450
  Stats: 73 lines in 4 files changed: 73 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1403.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1403/head:pull/1403

PR: https://git.openjdk.org/skara/pull/1403