chore(docs): Fix CHANGELOG tracebacks codeblocks

With v5.1.0 CHANGELOG.md was updated that mangled v1.10.0 triple
backtick codeblock Traceback output that made sphinx fail [1] with a
non-zero return code.

The resulting docs appears to be processes as text after the failing
line [2].  While reviewing other backtick codeblocks fix v1.8.0 [3] to
the original traceback.

[1] https://github.com/python-gitlab/python-gitlab/actions/runs/12060608158/job/33631303063#step:5:204
[2] https://python-gitlab.readthedocs.io/en/v5.1.0/changelog.html#v1-10-0-2019-07-22
[3] https://python-gitlab.readthedocs.io/en/v5.0.0/changelog.html#id258