The runner can send invalid UTF-8 job logs. For example, when a job log
is truncated to the default 4 MB limit, a multi-byte UTF-8 character can
be cut off prematurely. A similar issue could occur if multi-byte UTF-8
characters are streamed across chunk boundaries.

To avoid showing a 500 error, we previously forced the data to UTF-8
mode, but we didn't do anything about invalid UTF-8 characters. We now
replace the invalid UTF-8 with the default unknown character so the job
log can be viewed.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/330311

Changelog: fixed